Ever had a (Drupal) website that just did not respond fast enough and you could not figure out what was causing it? I bet you did not look at the frontend code. The part which takes place in the browser of the site visitor. As a web developer we all tend to have big and (resource) heavy machines with powerful CPU's. Most of the time that's not the case for the visitor of your website. Even worse, the visitor might be using IE7 or older!