 <p>The speed of the JavaScript engine depends on several factors, including the code itself. Some code can be optimized for specific browsers, although developers aren't supposed to do such a thing.</p> <p>Does it matter? It sure does! With the current Web 2.0 developments, where we have JavaScript doing all kinds of Ajax things, speed suddenly becomes important. Even this site uses JavaScript, even though it's just to notify me that another answer has posted while I was typing this message.</p> <p>Most browsers have their own engine and they're competing very hard with one another to get the best performance. The fastest one? Undetermined since most comparisons have been a bit colored in favor of the company that sponsored the comparison.</p> <p>Still, a fast engine is useless when the code is written in a bad way...</p>