One of the gems of Apple’s Safari web browser is its Web Inspector feature. Web Inspector, only available after turning on the development tab, displays stylesheets, images, and scripts on any webpage. However, the most useful part of Web Inspector is its Network feature, which shows in real-time the order and speed in which documents and scripts transfer from the server to the browser. Use it to find which scripts, files, or images are placing the most strain on browsers. Then fix it.

http://www.apple.com/safari/