Google's Page Speed

Google has released a Firefox add-on called Page Speed. It integrates with another add-on, Firebug, and is aimed at web developers trying to make their pages faster. “Page Speed is a tool we’ve been using internally to improve the performance of our web pages,” 

Also see YSlow, another Firebug add-on (this time by Yahoo) that “analyzes web pages and suggests ways to improve their performance”.