Dries Buytaert


Yahoo! released YSlow, a Firefox extension that integrates with the popular Firebug tool. YSlow was originally developed as an internal tool at Yahoo! with the help of Steve Souders, Chief Performance at Yahoo! and author of O'Reilly's High Performance Websites book.

YSlow analyzes the front-end performance of your website and tells you why it might be slow. For each component of a page (images, scripts, stylesheets) it checks its size, whether it was gzipped, the Expires-header, the ETag-header, etc. YSlow takes all this information into account and computes a performance grade for the page you are analyzing.

The current YSlow score for the drupal.org front page is 74 (C). YSlow suggests that we reduce the number of CSS background images using CSS sprites, that we use a Content Delivery Network (CDN) like Akamai for delivering static files, and identifies an Apache configuration issue that affects the Entity Tags or ETags of static files. The problem is that, by default, Apache constructs ETags using attributes that make them unique to a specific server. A stock Apache embeds inode numbers in the ETag which dramatically reduces the odds of the validity test succeeding on web sites with multiple servers; the ETags won't match when a browser gets the original component from server A and later tries to validate that component on server B.

Here are some other YSlow scores (higher is better):

From what I have seen, Apache configuration issues, and not CMS implementation issues, are the main source of low YSlow scores. Be careful not to draw incorrect conclusions from these numbers; they are often not representative for the CMS software itself.

And it doesn't change the fact that drupal.org is currently a lot slower than most of these other sites. That is explained by drupal.org's poor back-end performance, and not by the front-end performance as measured by YSlow. (We're working on adding a second database server to drupal.org.)

— Dries Buytaert