Just for fun, I tested JavaScript performance of several web browsers, using the well-known SunSpider benchmark tool. The test machine is a fairly old Fujitsu-Siemens Amilo notebook with AMD Turion64 1.8 GHz processor and 1 GB RAM, running Microsoft Windows XP SP2.
The result is not surprising. Internet Explorer is notoriously slow but there is hope with IE8. Mozilla developers have done a great job optimizing Firefox. WebKit with SquirrelFish (and surely the upcoming Safari 4) really shines in speed.