Using YSlow to Optimize Web Site Performance Continued

The second part of an article/tutorial on using the YSlow firebug extension to optimize web site performance.

This article is a continuation of a previous article. If you haven’t yet, read the previous article: Using YSlow to Optimize Web Site Performance.

In this post, I’ll  cover rules 5-13 and summarize the results of the optimizations that I made to my site.

YSlow’s 13 Rules to Improve Web Site Performance (rules 5-13)

A number of these rules are fairly simple, so I’ll cover some together.

5. Put CSS at the top

6. Put JS at the bottom

These rules are fairly easy to follow and most sites shouldn’t have to change anything. Putting CSS in the document <head> makes the page appear to load faster because of progressive rendering. It turns out that javascript blocks parallel downloads, so ideally you want to load javascript when everything else has already loaded. Also, if the javascript is hosted externally and the external server is slow, the rest of the page should load without problems.

7. Avoid CSS expressions

I’ll be honest here. I didn’t even know CSS had expressions until I saw this rule. YSlow recommends avoiding them because they are evaluated an absurd number of times.

8. Make JS and CSS external

External files can be cached indepently. If your users usually browse to more than one page on your site or you have frequent returning visitors, this should improve load times.

9. Reduce DNS lookups

DNS lookups can still take a fair amount of time even on a broadband connection. Basically, try to keep the number of unique host names fairly low. There is one competing performance benefit to serving page components from multiple hosts. It turns out that most browsers will only make two simultaneous connections to a given host. So, serving content from multiple hosts can speed up page load times especially if there are a lot of components.

10. Minify JS and CSS

Actually, in YSlow this rule says Minify JS, but on the explanation page, they added CSS. Minifying these files simply makes them smaller even if you’re already compressing them. The best minifier I found is the YUI Compressor, which handles both CSS and javascript. Here are the results of minifying my site’s stylesheet:

Uncompressed gzipped
Original 9814 bytes 2806 bytes
Minified 6164 bytes 1645 bytes
Savings 37.2% 41.4%

The YUI Compressor is pretty easy to use, but using it is going to get annoying unless you automate. To solve this problem, I once again modified my publish CSS script to do the minifying automatically. If you read the first part of this article and you’re wondering how many times I’m going to modify my publish script, this is the final version:



cat	style.css.original \
	../../plugins/wp-recaptcha/recaptcha.css \
	../../plugins/deko-boko-a-recaptcha-contact-form-plugin/display/dekoboko.css \
	| java -jar ${YUICOMPRESSOR_PATH} --type css > style-${SERIAL}.css

sed "s/REPLACE_WITH_SERIAL/${SERIAL}/g" < header.php.original > header.php

rm style-${OLD_SERIAL}.css


Using publish scripts for stylesheets and javascript might seem like overkill at first, but that simple script has allowed me to make significant improvements that otherwise would be too time consuming to implement.

11. Avoid redirects

12. Remove duplicate scripts

These two rules are fairly easy to follow. Sometimes you can’t avoid redirects such as when you move to a new domain or restructure your site. Those sort of redirects are good and they maintain the reputation your site has built with search engines. That said, a lot of redirects can be eliminated simply by adding a trailing slash to a URL. Be especially mindful if you’re working on hand-coded sites rather than CMS-based sites. Remove duplicate scripts is pretty obvious. Unfortunately, Google Adsense serves the same scripts for each ad on a page and there’s no way to fix it.

13. Configure ETags

I won’t say much about ETags. This YSlow rule actually means configure ETags or remove them entirely. For a site being served off a single web server, ETags offer no benefits and some drawbacks including the fact that Apache generates invalid ETags for gzipped content. I recommend turning them off unless you’re willing to configure them per Yahoo’s ETags best practices. To disable ETags in Apache add the following to the virtual host configuration or the main .htaccess:

FileETag None


Finally, here are the results that I obtained. First, my YSlow score:

Final YSlow Performance Tab
Final YSlow Performance Tab

I’m somewhat disappointed that after all of that, my score barely improved, moving from an F (59) to a D (63). Unfortunately, most of the remaining areas of optimization are either unrealistic as in Use a CDN or are out of my control.

What about real performance? The following tables include the minimum, maximum, and average times out of 10 page loads. First, with a cold cache:

Page Load Times Before and After Optimization (cold cache)
Minimum Maximum Average
Before Optimization 1.243s 1.890s 1.422s
After Optimization 1.024s 1.410s 1.215s
Speed Increase 17.6% 25.4% 14.6%

I’m actually fairly satisfied. Most of the optimizations I did would benefit repeat visitors rather than new visitors. A 15% performance improvement is not bad at all. Next, with a warm cache:

Page Load Times Before and After Optimization (warm cache)
Minimum Maximum Average
Before Optimization 0.817s 1.176s 0.968s
After Optimization 0.732s 0.920s 0.815s
Speed Increase 10.4% 21.8% 15.8%

These results baffled me. The improvements are only slightly better than the cold cache numbers and I really thought page loads would speed up more for repeat visitors. After some investigation, it turns out that I was loading pages in the wrong way. Specifically, I was hitting the f5 key to reload the page in my warm cache tests. Reloading with f5 has a special meaning in at least Firefox. It tells the browser to check that every item in the cache is the correct version even if the item is set to expire in 10 years. Here are the numbers without using f5:

Non-f5 Page Load Times After Optimization (warm cache)
Minimum Maximum Average
0.397s 0.525s 0.448s

Now, those are some fast page loads. I’m not interested enough to revert all my changes and re-test, but some statistics from YSlow suggest that I probably sped up warm cache page loads by a fair amount. Here’s the initial Stats tab, prior to any optimization:

Initial YSlow Stats Tab
Initial YSlow Stats Tab

And here’s the final Stats tab after all the optimizations:

Final YSlow Stats Tab
Final YSlow Stats Tab

Of particular note: in the case of a primed cache, I reduced the number of HTTP requests from 25 to 15.

Overall, I’m satisfied with the performance improvements. Even if the improvements are not amazing, working with YSlow is not particularly hard or time consuming. In fact, writing this article took much longer than the actual changes I made to my web site. A 15% reduction in load times is worth an hour or two of writing scripts and changing server configurations.

Using YSlow to Optimize Web Site Performance

An article/tutorial on using the YSlow firebug extension to optimize web site performance.

I’m a big fan of Google Tech Talks. Recently, I caught a particularly interesting one from Steve Souders called Life’s Too Short – Write Fast Code. Steve Souders used to work for Yahoo! as their chief web performance guru; he now does much the same at Google. The talk was a continuation of a previous lecture on a firebug extension that he wrote called YSlow. Suffice it to say, he knows what he’s talking about when it comes to high performance web sites. Anyway, it piqued my interest, so I decided to try to improve the performance of my own site. This article/tutorial and the next chronicle the changes I made and the results that I obtained.

YSlow’s 13 Rules to Improve Web Site Performance (rules 1-4)

YSlow grades web sites based on 13 different rules. Browse to your site, click on the YSlow icon in the lower right corner of Firefox, and click on the Performance tab. Here’s how one post on my site ranked before making any changes:

Initial YSlow Performance Tab
Initial YSlow Performance Tab

Clicking on any rule takes you to a page explaining it in more detail.

A quick note: the screenshot above was taken before I disabled Woopra, which is web analytics software similar to Google Analytics. I had tried it out a couple months ago and had simply forgotten that I had it enabled. Disabling Woopra improved my grades very slighty in a number of rules, but I don’t include that optimization in the main article because it’s technically a functional change to my site. As explained below, YSlow doesn’t make functional recommendations.

What follows are the changes that I made to my site, rule-by-rule.

1. Make fewer HTTP requests

This is one of the most important rules and it’s also one of the most difficult to implement. It’s worth mentioning at this point that YSlow’s general strategy isn’t to recommend changes that alter the actual content and functionality of a site, but instead suggest changes that will make the same content load faster. In other words, if you have a gallery with 20 images, the recommendation isn’t to cut the gallery down to 5 images. Consequently, Make fewer HTTP requests focuses on three components of web pages: javascript, stylesheets, and background images.

Expanding the rule will show counts of each component if they exceed YSlow’s thresolds:

Make fewer requests expanded
Make fewer requests expanded

In the case of my blog, the problem comes down to external javascript and CSS. To get a list of page components, click on the Components tab. They’re sorted by type so, it’s very easy to identify the problem files.

Javascript and CSS components
Javascript and CSS components

I can group these into 4 basic categories: Google Adsense, Google Analytics, reCAPTCHA, and WordPress plugins. Adsense has a particularly egregious number of external javascript files. Unfortunately, there’s nothing I can do about it as modifying the code is against Adsense policies. Analytics has just one javascript file and keeping that external is probably a good idea. Almost anyone who browses more than a handful of sites will have ga.js cached as Analytics use is pervasive. Similarly, there’s a good possibility that visitors will have the main reCAPTCHA javascript cached and the other code is dynamic so you don’t want to mess with it. Finally, there are the stylesheets from WordPress plugins.

Combining CSS into one file–especially when it’s all hosted on the same server–is a really good idea. In this case, I’m serving my main style.css file and two other stylesheets from the wp-recaptcha plugin and the Deko Boko plugin. I wrote a simple script to combine all of the files into one, like so:


cat style.css.original \
    ../../plugins/wp-recaptcha/recaptcha.css \
    ../../plugins/deko-boko-a-recaptcha-contact-form-plugin/display/dekoboko.css \
    > style.css

I also modified both plugins to no longer include a link to their original stylesheets in the <head> section of the page. Search the PHP files for references to the original stylesheets and comment those lines out.

Before moving on, it’s worth mentioning that you should use CSS sprites if at all possible. Many sites use tens of icons per page. Combining those tiny icons into one sprite is a huge performance boost.

2. Use a CDN

One persistent criticism of YSlow is that it focuses on performance improvements to major sites. I don’t really agree with that; in fact, nearly every rule applies equally to both small and large sites. Use a CDN is the exception. A CDN allows you to serve content to users from geographically close servers. It’s a decent performance improvement, but it’s also incredibly expensive.

3. Add an Expires header

What’s the fastest HTTP request? One that never takes place. The idea behind far future Expires headers is that you should allow permanent caching of any page component that won’t change or even that is unlikely to change very often. If it does need to be modified, you simply rename it. To accomplish this using Apache, enable mod_expires and add the following to the virtual host configuration or the main .htaccess:

<IfModule mod_expires.c>
    ExpiresActive On
    ExpiresByType image/gif "access plus 10 years"
    ExpiresByType image/jpeg "access plus 10 years"
    ExpiresByType image/png "access plus 10 years"
    ExpiresByType text/css "access plus 10 years"

Warning: The preceding configuration means that any time you change an image or stylesheet served my Apache, you must rename it. If a browser or proxy server has the component cached, it will never check to see if its version is the same as the version on your server. For images, this is probably not an annoyance; changing an image usually means uploading a new version in which case it will have a different file name anyway. For stylesheets, this can be annoying if you don’t automate the process.

I solved the stylesheet problem with a script. In the same directory as style.css, I created a text file like so:

$ echo 0 > serial.txt

Next, I modified the script that combined the 3 original stylesheets I was using like so:



cat style.css.original \
    ../../plugins/wp-recaptcha/recaptcha.css \
    ../../plugins/deko-boko-a-recaptcha-contact-form-plugin/display/dekoboko.css \
    > style-${SERIAL}.css

sed "s/REPLACE_WITH_SERIAL/${SERIAL}/g" < header.php.original > header.php

rm -f style-${OLD_SERIAL}.css


The script reads a number in from serial.txt, increments it, creates a new stylesheet with the number appended, and in this case modifies WordPress’ header.php to link to the new stylesheet. Every time I want a different style on my site, I modifiy style.css.original and run this script.

4. Gzip components

Everyone in every circumstance should enable mod_deflate. Way back in the early days of the web there were browsers that didn’t play well with gzip; they basically don’t exist anymore. In Apache, enable mod_deflate–it’s probably already enabled–and add the following to the virtual host configuration or the main .htacess:

<IfModule mod_deflate.c>
    AddOutputFilterByType DEFLATE text/html text/plain text/xml text/css

Continued Soon

When I started writing this post, I didn’t realize how long it was going to be. Part two, which will include the other 9 easier-to-implement and less important rules, should be up within a few days. I’ll also include some before-and-after performance statistics.

Part two is now finished: Using YSlow to Optimize Web Site Performance Continued.