YSlow is a tool developed by Yahoo! and it examines all the components of your page's performance and offers suggestions for improvements. YSlow is integrated into the Firebug web development tool for Firefox, and for Chrome you can download their extension.
The Yahoo! team has identified 34 rules that affect a web page's performance and YSlow is based on 23 of these 34 rules. Studies have shown that if a developer can improve the following 23 rules, they can enhance web performance from up to 25 to 50 percent.