Site slower with CDN? Check your CSS!

A couple of days ago we implemented a CDN for a Drupal-based website, using MaxCDN/ NetDNA and the Drupal CDN module. We were very surprised to discover the site became … slower. It took us some time to identify the problem, but CSS turned out to be the culprit;

  1. The supplier created separate CSS-file for each and every template using SASS, causing some automated duplication of CSS
  2. Some of that CSS was not in the theme .info-file and wasn’t added using drupal_add_css either, but the link was hardcoded in the template-file
  3. The CSS that was added the normal way (i.e. not hardcoded in the template) was picked up and modified by the CDN module (changing paths for e.g. images and fonts into URL’s pointing to the CDN)
  4. CSS that was hardcoded in the template was not visible for the CDN-module, so the paths were not updated and still pointed to the origin webserver
  5. Because of duplication of e.g. background-images and fonts, pointing at both the CDN and the origin-server, these assets were downloaded twice and the extra file-size resulted in a site that was slower with than without a CDN

Once we understood the problem, the solution was pretty simple;

  1. clean up the CSS, avoiding to re-declare e.g. @font-face in multiple templates (resulting in a smaller CSS file download size as well)
  2. add CSS using drupal_add_css (with the option not to aggregate, as IE8 might choke on the massive amount of CSS) for the CDN module to take that into account as well

Web Performance Optimization can be so much fun!

Leave a Reply

Your email address will not be published. Required fields are marked *