The Exceptional Performance team has identified a number of best practices for making web pages fast.
The list includes 35 best practices divided into 7 categories.
Minimize HTTP Requests
tag: content
80% of the end-user response time is spent on the front-end. Most of
this time is tied up in downloading all the components in the page:
images, stylesheets, scripts, Flash, etc. Reducing the number of
components in turn reduces the number of HTTP requests required to
render the page. This is the key to faster pages.
One way to reduce the number of components in the page is to simplify
the page's design. But is there a way to build pages with richer
content while also achieving fast response times? Here are some
techniques for reducing the number of HTTP requests, while still
supporting rich page designs.
Combined files are a way to reduce the number of
HTTP requests by combining all scripts into a single script, and
similarly combining all CSS into a single stylesheet. Combining files is
more challenging when the scripts and stylesheets vary from page to
page, but making this part of your release process improves response
times.
CSS Sprites
are the preferred method for reducing the number of image requests.
Combine your background images into a single image and use the CSS
background-image and
background-position properties to display the desired image segment.
Image maps
combine multiple images into a single image. The overall size is about
the same, but reducing the number of HTTP requests speeds up the page.
Image maps only work if the images are contiguous in the page, such as a
navigation bar. Defining the coordinates of image maps can be tedious
and error prone. Using image maps for navigation is not accessible too,
so it's not recommended.
Inline images use the
data: URL scheme
to embed the image data in the actual page. This can increase the size
of your HTML document. Combining inline images into your (cached)
stylesheets is a way to reduce HTTP requests and avoid increasing the
size of your pages. Inline images are not yet supported across all major
browsers.
Reducing the number of HTTP requests in your page is the place to
start. This is the most important guideline for improving performance
for first time visitors. As described in Tenni Theurer's blog post
Browser Cache Usage - Exposed!,
40-60% of daily visitors to your site come in with an empty cache.
Making your page fast for these first time visitors is key to a better
user experience.
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Use a Content Delivery Network
tag: server
The user's proximity to your web server has an impact on response
times. Deploying your content across multiple, geographically dispersed
servers will make your pages load faster from the user's perspective.
But where should you start?
As a first step to implementing geographically dispersed content,
don't attempt to redesign your web application to work in a distributed
architecture. Depending on the application, changing the architecture
could include daunting tasks such as synchronizing session state and
replicating database transactions across server locations. Attempts to
reduce the distance between users and your content could be delayed by,
or never pass, this application architecture step.
Remember that 80-90% of the end-user response time is spent
downloading all the components in the page: images, stylesheets,
scripts, Flash, etc. This is the
Performance Golden Rule.
Rather than starting with the difficult task of redesigning your
application architecture, it's better to first disperse your static
content. This not only achieves a bigger reduction in response times,
but it's easier thanks to content delivery networks.
A content delivery network (CDN) is a collection of web servers
distributed across multiple locations to deliver content more
efficiently to users. The server selected for delivering content to a
specific user is typically based on a measure of network proximity. For
example, the server with the fewest network hops or the server with the
quickest response time is chosen.
Some large Internet companies own their own CDN, but it's cost-effective to use a CDN service provider, such as
Akamai Technologies,
EdgeCast, or
level3.
For start-up companies and private web sites, the cost of a CDN service
can be prohibitive, but as your target audience grows larger and
becomes more global, a CDN is necessary to achieve fast response times.
At Yahoo!, properties that moved static content off their application
web servers to a CDN (both 3rd party as mentioned above as well as
Yahoo’s own
CDN)
improved end-user response times by 20% or more. Switching to a CDN is a
relatively easy code change that will dramatically improve the speed of
your web site.
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Add an Expires or a Cache-Control Header
tag: server
There are two aspects to this rule:
- For static components: implement "Never expire" policy by setting far future
Expires header
- For dynamic components: use an appropriate
Cache-Control header to help the browser with conditional requests
Web page designs are getting richer and richer, which means more
scripts, stylesheets, images, and Flash in the page. A first-time
visitor to your page may have to make several HTTP requests, but by
using the Expires header you make those components cacheable. This
avoids unnecessary HTTP requests on subsequent page views. Expires
headers are most often used with images, but they should be used on
all components including scripts, stylesheets, and Flash components.
Browsers (and proxies) use a cache to reduce the number and size of
HTTP requests, making web pages load faster. A web server uses the
Expires header in the HTTP response to tell the client how long a
component can be cached. This is a far future Expires header, telling
the browser that this response won't be stale until April 15, 2010.
Expires: Thu, 15 Apr 2010 20:00:00 GMT
If your server is Apache, use the ExpiresDefault directive to set an
expiration date relative to the current date. This example of the
ExpiresDefault directive sets the Expires date 10 years out from the
time of the request.
ExpiresDefault "access plus 10 years"
Keep in mind, if you use a far future Expires header you have to
change the component's filename whenever the component changes. At
Yahoo! we often make this step part of the build process: a version
number is embedded in the component's filename, for example,
yahoo_2.0.6.js.
Using a far future Expires header affects page views only after a
user has already visited your site. It has no effect on the number of
HTTP requests when a user visits your site for the first time and the
browser's cache is empty. Therefore the impact of this performance
improvement depends on how often users hit your pages with a primed
cache. (A "primed cache" already contains all of the components in the
page.) We
measured this at Yahoo!
and found the number of page views with a primed cache is 75-85%. By
using a far future Expires header, you increase the number of components
that are cached by the browser and re-used on subsequent page views
without sending a single byte over the user's Internet connection.
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Gzip Components
tag: server
The time it takes to transfer an HTTP request and response across the
network can be significantly reduced by decisions made by front-end
engineers. It's true that the end-user's bandwidth speed, Internet
service provider, proximity to peering exchange points, etc. are beyond
the control of the development team. But there are other variables that
affect response times. Compression reduces response times by reducing
the size of the HTTP response.
Starting with HTTP/1.1, web clients indicate support for compression with the Accept-Encoding header in the HTTP request.
Accept-Encoding: gzip, deflate
If the web server sees this header in the request, it may compress
the response using one of the methods listed by the client. The web
server notifies the web client of this via the Content-Encoding header
in the response.
Content-Encoding: gzip
Gzip is the most popular and effective compression method at this time. It was developed by the GNU project and standardized by
RFC 1952. The only other compression format you're likely to see is deflate, but it's less effective and less popular.
Gzipping generally reduces the response size by about 70%.
Approximately 90% of today's Internet traffic travels through browsers
that claim to support gzip. If you use Apache, the module configuring
gzip depends on your version: Apache 1.3 uses
mod_gzip while Apache 2.x uses
mod_deflate.
There are known issues with browsers and proxies that may cause a
mismatch in what the browser expects and what it receives with regard to
compressed content. Fortunately, these edge cases are dwindling as the
use of older browsers drops off. The Apache modules help out by adding
appropriate Vary response headers automatically.
Servers choose what to gzip based on file type, but are typically too
limited in what they decide to compress. Most web sites gzip their HTML
documents. It's also worthwhile to gzip your scripts and stylesheets,
but many web sites miss this opportunity. In fact, it's worthwhile to
compress any text response including XML and JSON. Image and PDF files
should not be gzipped because they are already compressed. Trying to
gzip them not only wastes CPU but can potentially increase file sizes.
Gzipping as many file types as possible is an easy way to reduce page weight and accelerate the user experience.
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Put Stylesheets at the Top
tag: css
While researching performance at Yahoo!, we discovered that moving stylesheets to the document HEAD makes pages
appear to be loading faster. This is because putting stylesheets in the HEAD allows the page to render progressively.
Front-end engineers that care about performance want a page to load
progressively; that is, we want the browser to display whatever content
it has as soon as possible. This is especially important for pages with a
lot of content and for users on slower Internet connections. The
importance of giving users visual feedback, such as progress indicators,
has been well researched and
documented.
In our case the HTML page is the progress indicator! When the browser
loads the page progressively the header, the navigation bar, the logo at
the top, etc. all serve as visual feedback for the user who is waiting
for the page. This improves the overall user experience.
The problem with putting stylesheets near the bottom of the document
is that it prohibits progressive rendering in many browsers, including
Internet Explorer. These browsers block rendering to avoid having to
redraw elements of the page if their styles change. The user is stuck
viewing a blank white page.
The
HTML specification
clearly states that stylesheets are to be included in the HEAD of the
page: "Unlike A, [LINK] may only appear in the HEAD section of a
document, although it may appear any number of times." Neither of the
alternatives, the blank white screen or flash of unstyled content, are
worth the risk. The optimal solution is to follow the HTML specification
and load your stylesheets in the document HEAD.
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Put Scripts at the Bottom
tag: javascript
The problem caused by scripts is that they block parallel downloads. The
HTTP/1.1 specification
suggests that browsers download no more than two components in parallel
per hostname. If you serve your images from multiple hostnames, you can
get more than two downloads to occur in parallel. While a script is
downloading, however, the browser won't start any other downloads, even
on different hostnames.
In some situations it's not easy to move scripts to the bottom. If, for example, the script uses
document.write
to insert part of the page's content, it can't be moved lower in the
page. There might also be scoping issues. In many cases, there are ways
to workaround these situations.
An alternative suggestion that often comes up is to use deferred scripts. The
DEFER
attribute indicates that the script does not contain document.write,
and is a clue to browsers that they can continue rendering.
Unfortunately, Firefox doesn't support the
DEFER attribute.
In Internet Explorer, the script may be deferred, but not as much as
desired. If a script can be deferred, it can also be moved to the bottom
of the page. That will make your web pages load faster.
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Avoid CSS Expressions
tag: css
CSS expressions are a powerful (and dangerous) way to set CSS
properties dynamically. They were supported in Internet Explorer
starting with version 5, but were
deprecated starting with IE8. As an example, the background color could be set to alternate every hour using CSS expressions:
background-color: expression( (new Date()).getHours()%2 ? "#B8D4FF" : "#F08A00" );
As shown here, the
expression method accepts a JavaScript expression. The CSS property is set to the result of evaluating the JavaScript expression. The
expression
method is ignored by other browsers, so it is useful for setting
properties in Internet Explorer needed to create a consistent experience
across browsers.
The problem with expressions is that they are evaluated more
frequently than most people expect. Not only are they evaluated when the
page is rendered and resized, but also when the page is scrolled and
even when the user moves the mouse over the page. Adding a counter to
the CSS expression allows us to keep track of when and how often a CSS
expression is evaluated. Moving the mouse around the page can easily
generate more than 10,000 evaluations.
One way to reduce the number of times your CSS expression is
evaluated is to use one-time expressions, where the first time the
expression is evaluated it sets the style property to an explicit value,
which replaces the CSS expression. If the style property must be set
dynamically throughout the life of the page, using event handlers
instead of CSS expressions is an alternative approach. If you must use
CSS expressions, remember that they may be evaluated thousands of times
and could affect the performance of your page.
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Make JavaScript and CSS External
tag: javascript, css
Many of these performance rules deal with how external components are
managed. However, before these considerations arise you should ask a
more basic question: Should JavaScript and CSS be contained in external
files, or inlined in the page itself?
Using external files in the real world generally produces faster
pages because the JavaScript and CSS files are cached by the browser.
JavaScript and CSS that are inlined in HTML documents get downloaded
every time the HTML document is requested. This reduces the number of
HTTP requests that are needed, but increases the size of the HTML
document. On the other hand, if the JavaScript and CSS are in external
files cached by the browser, the size of the HTML document is reduced
without increasing the number of HTTP requests.
The key factor, then, is the frequency with which external JavaScript
and CSS components are cached relative to the number of HTML documents
requested. This factor, although difficult to quantify, can be gauged
using various metrics. If users on your site have multiple page views
per session and many of your pages re-use the same scripts and
stylesheets, there is a greater potential benefit from cached external
files.
Many web sites fall in the middle of these metrics. For these sites,
the best solution generally is to deploy the JavaScript and CSS as
external files. The only exception where inlining is preferable is with
home pages, such as
Yahoo!'s front page and
My Yahoo!.
Home pages that have few (perhaps only one) page view per session may
find that inlining JavaScript and CSS results in faster end-user
response times.
For front pages that are typically the first of many page views,
there are techniques that leverage the reduction of HTTP requests that
inlining provides, as well as the caching benefits achieved through
using external files. One such technique is to inline JavaScript and CSS
in the front page, but dynamically download the external files after
the page has finished loading. Subsequent pages would reference the
external files that should already be in the browser's cache.
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Reduce DNS Lookups
tag: content
The Domain Name System (DNS) maps hostnames to IP addresses, just as
phonebooks map people's names to their phone numbers. When you type
www.yahoo.com into your browser, a DNS resolver contacted by the browser
returns that server's IP address. DNS has a cost. It typically takes
20-120 milliseconds for DNS to lookup the IP address for a given
hostname. The browser can't download anything from this hostname until
the DNS lookup is completed.
DNS lookups are cached for better performance. This caching can occur
on a special caching server, maintained by the user's ISP or local area
network, but there is also caching that occurs on the individual user's
computer. The DNS information remains in the operating system's DNS
cache (the "DNS Client service" on Microsoft Windows). Most browsers
have their own caches, separate from the operating system's cache. As
long as the browser keeps a DNS record in its own cache, it doesn't
bother the operating system with a request for the record.
Internet Explorer caches DNS lookups for 30 minutes by default, as specified by the
DnsCacheTimeout registry setting. Firefox caches DNS lookups for 1 minute, controlled by the
network.dnsCacheExpiration configuration setting. (Fasterfox changes this to 1 hour.)
When the client's DNS cache is empty (for both the browser and the
operating system), the number of DNS lookups is equal to the number of
unique hostnames in the web page. This includes the hostnames used in
the page's URL, images, script files, stylesheets, Flash objects, etc.
Reducing the number of unique hostnames reduces the number of DNS
lookups.
Reducing the number of unique hostnames has the potential to reduce
the amount of parallel downloading that takes place in the page.
Avoiding DNS lookups cuts response times, but reducing parallel
downloads may increase response times. My guideline is to split these
components across at least two but no more than four hostnames. This
results in a good compromise between reducing DNS lookups and allowing a
high degree of parallel downloads.
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Minify JavaScript and CSS
tag: javascript, css
Minification is the practice of removing unnecessary characters from
code to reduce its size thereby improving load times. When code is
minified all comments are removed, as well as unneeded white space
characters (space, newline, and tab). In the case of JavaScript, this
improves response time performance because the size of the downloaded
file is reduced. Two popular tools for minifying JavaScript code are
JSMin and
YUI Compressor. The YUI compressor can also minify CSS.
Obfuscation is an alternative optimization that can be applied to
source code. It's more complex than minification and thus more likely to
generate bugs as a result of the obfuscation step itself. In a survey
of ten top U.S. web sites, minification achieved a 21% size reduction
versus 25% for obfuscation. Although obfuscation has a higher size
reduction, minifying JavaScript is less risky.
In addition to minifying external scripts and styles, inlined
An alternative in PHP would be to create a function called insertScript.
In addition to preventing the same script from being inserted
multiple times, this function could handle other issues with scripts,
such as dependency checking and adding version numbers to script
filenames to support far future Expires headers.
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tag: server
Entity tags (ETags) are a mechanism that web servers and browsers use
to determine whether the component in the browser's cache matches the
one on the origin server. (An "entity" is another word a "component":
images, scripts, stylesheets, etc.) ETags were added to provide a
mechanism for validating entities that is more flexible than the
last-modified date. An ETag is a string that uniquely identifies a
specific version of a component. The only format constraints are that
the string be quoted. The origin server specifies the component's ETag
using the ETag response header.
HTTP/1.1 200 OK
Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT
ETag: "10c24bc-4ab-457e1c1f"
Content-Length: 12195
Later, if the browser has to validate a component, it uses the If-None-Match
header to pass the ETag back to the origin server. If the ETags match, a
304 status code is returned reducing the response by 12195 bytes for
this example.
GET /i/yahoo.gif HTTP/1.1
Host: us.yimg.com
If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT
If-None-Match: "10c24bc-4ab-457e1c1f"
HTTP/1.1 304 Not Modified
The problem with ETags is that they typically are constructed using
attributes that make them unique to a specific server hosting a site.
ETags won't match when a browser gets the original component from one
server and later tries to validate that component on a different server,
a situation that is all too common on Web sites that use a cluster of
servers to handle requests. By default, both Apache and IIS embed data
in the ETag that dramatically reduces the odds of the validity test
succeeding on web sites with multiple servers.
The ETag format for Apache 1.3 and 2.x is inode-size-timestamp.
Although a given file may reside in the same directory across multiple
servers, and have the same file size, permissions, timestamp, etc., its
inode is different from one server to the next.
IIS 5.0 and 6.0 have a similar issue with ETags. The format for ETags on IIS is Filetimestamp:ChangeNumber. A ChangeNumber is a counter used to track configuration changes to IIS. It's unlikely that the ChangeNumber is the same across all IIS servers behind a web site.
The end result is ETags generated by Apache and IIS for the exact
same component won't match from one server to another. If the ETags
don't match, the user doesn't receive the small, fast 304 response that
ETags were designed for; instead, they'll get a normal 200 response
along with all the data for the component. If you host your web site on
just one server, this isn't a problem. But if you have multiple servers
hosting your web site, and you're using Apache or IIS with the default
ETag configuration, your users are getting slower pages, your servers
have a higher load, you're consuming greater bandwidth, and proxies
aren't caching your content efficiently. Even if your components have a
far future Expires header, a conditional GET request is still made whenever the user hits Reload or Refresh.
If you're not taking advantage of the flexible validation model
that ETags provide, it's better to just remove the ETag altogether. The Last-Modified
header validates based on the component's timestamp. And removing the
ETag reduces the size of the HTTP headers in both the response and
subsequent requests. This Microsoft Support article describes how to remove ETags. In Apache, this is done by simply adding the following line to your Apache configuration file:
FileETag none
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Make Ajax Cacheable
tag: content
One of the cited benefits of Ajax is that it provides instantaneous
feedback to the user because it requests information asynchronously from
the backend web server. However, using Ajax is no guarantee that the
user won't be twiddling his thumbs waiting for those asynchronous
JavaScript and XML responses to return. In many applications, whether or
not the user is kept waiting depends on how Ajax is used. For example,
in a web-based email client the user will be kept waiting for the
results of an Ajax request to find all the email messages that match
their search criteria. It's important to remember that "asynchronous"
does not imply "instantaneous".
To improve performance, it's important to optimize these Ajax
responses. The most important way to improve the performance of Ajax is
to make the responses cacheable, as discussed in Add an Expires or a Cache-Control Header. Some of the other rules also apply to Ajax:
Let's look at an example. A Web 2.0 email client might use Ajax to
download the user's address book for autocompletion. If the user hasn't
modified her address book since the last time she used the email web
app, the previous address book response could be read from cache if that
Ajax response was made cacheable with a future Expires or Cache-Control
header. The browser must be informed when to use a previously cached
address book response versus requesting a new one. This could be done by
adding a timestamp to the address book Ajax URL indicating the last
time the user modified her address book, for example, &t=1190241612.
If the address book hasn't been modified since the last download, the
timestamp will be the same and the address book will be read from the
browser's cache eliminating an extra HTTP roundtrip. If the user has
modified her address book, the timestamp ensures the new URL doesn't
match the cached response, and the browser will request the updated
address book entries.
Even though your Ajax responses are created dynamically, and might
only be applicable to a single user, they can still be cached. Doing so
will make your Web 2.0 apps faster.
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Flush the Buffer Early
tag: server
When users request a page, it can take anywhere from 200 to 500ms for the backend server to stitch together the HTML page.
During this time, the browser is idle as it waits for the data to arrive.
In PHP you have the function flush().
It allows you to send your partially ready HTML response to the browser so that
the browser can start fetching components while your backend is busy with the rest of the HTML page.
The benefit is mainly seen on busy backends or light frontends.
A good place to consider flushing is right after the HEAD because the HTML for the head is
usually easier to produce and it allows you to include any CSS and JavaScript
files for the browser to start fetching in parallel while the backend is still processing.
Example:
...
...
Yahoo! search pioneered research and real user testing to prove the benefits of using this technique.
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Use GET for AJAX Requests
tag: server
The Yahoo! Mail team found that when using XMLHttpRequest,
POST is implemented in the browsers as a two-step process:
sending the headers first, then sending data. So it's best to use
GET, which only takes one TCP packet to send (unless you have a lot of
cookies).
The maximum URL length in IE is 2K, so if you send more than 2K data
you might not be able to use GET.
An interesting side affect is that POST without actually posting any data behaves like GET.
Based on the HTTP specs,
GET is meant for retrieving information, so it
makes sense (semantically) to use GET when you're only
requesting data, as opposed to sending data to be stored server-side.
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Post-load Components
tag: content
You can take a closer look at your page and ask yourself: "What's
absolutely required in order to render the page initially?".
The rest of the content and components can wait.
JavaScript is an ideal candidate for splitting before and after the
onload event. For example
if you have JavaScript code and libraries that do drag and drop and
animations, those can wait,
because dragging elements on the page comes after the initial
rendering.
Other places to look for candidates for post-loading include hidden
content (content that appears after a user action) and images below the
fold.
Tools to help you out in your effort: YUI Image Loader allows you to delay images
below the fold and the YUI Get utility is an easy way to include JS and CSS on the fly.
For an example in the wild take a look at Yahoo! Home Page with Firebug's Net Panel turned on.
It's good when the performance goals are inline with other
web development best practices. In this case, the idea of
progressive enhancement tells us that JavaScript, when supported, can
improve the user experience but you have to make sure the page works
even without JavaScript. So after you've made sure the page
works fine, you can enhance it with some post-loaded scripts that
give you more bells and whistles such as drag and drop and animations.
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Preload Components
tag: content
Preload may look like the opposite of post-load, but it actually has a different goal.
By preloading components you can take advantage of the time the browser is idle and request components
(like images, styles and scripts) you'll need in the future.
This way when the user visits the next page, you could have most of the components already in
the cache and your page will load much faster for the user.
There are actually several types of preloading:
- Unconditional preload - as soon as onload fires, you go ahead and fetch some extra components.
Check google.com for an example of how a sprite image is requested onload. This sprite image is
not needed on the google.com homepage, but it is needed on the consecutive search result page.
- Conditional preload - based on a user action you make an educated guess where the user is headed next and preload accordingly.
On search.yahoo.com you can see how some extra components are requested
after you start typing in the input box.
- Anticipated preload - preload in
advance before launching a redesign. It often happens after a redesign
that you hear:
"The new site is cool, but it's slower than before". Part of the
problem could be that the users were visiting your old site with a
full cache, but the new one is always an empty cache experience.
You can mitigate this side effect by preloading some
components before you even launched the redesign. Your old site
can use the time the browser is idle and request images and scripts
that will be used by the new site
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Reduce the Number of DOM Elements
tag: content
A complex page means more bytes to download and it also means slower DOM access in JavaScript. It makes a difference
if you loop through 500 or 5000 DOM elements on the page when you want to add an event handler for example.
A high number of DOM elements can be a symptom that there's something that should be improved with the markup
of the page without necessarily removing content.
Are you using nested tables for layout purposes?
Are you throwing in more
s only to fix layout issues?
Maybe there's a better and more semantically correct way to do your markup.
A great help with layouts are the
YUI CSS utilities:
grids.css can help you with the overall layout, fonts.css and reset.css
can help you strip away the browser's defaults formatting.
This is a chance to start fresh and think about your markup,
for example use
s only when it makes sense semantically, and not because it renders a new line.
The number of DOM elements is easy to test, just type in Firebug's console:
document.getElementsByTagName('*').length
And how many DOM elements are too many? Check other similar pages that have good markup.
For example the
Yahoo! Home Page is a pretty busy page and still under 700 elements (HTML tags).
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Split Components Across Domains
tag: content
Splitting components allows you to maximize parallel downloads. Make sure you're using
not more than 2-4 domains because of the DNS lookup penalty.
For example, you can host your HTML and dynamic content
on
www.example.org
and split static components between
static1.example.org and
static2.example.org
For more information check
"
Maximizing Parallel Downloads in the Carpool Lane" by Tenni Theurer and Patty Chi.
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Minimize the Number of iframes
tag: content
Iframes allow an HTML document to be inserted in the parent document.
It's important to understand how iframes work so they can be used effectively.
.
