Have you checked out Dromaeo.com?  It’s a Mozilla owned site that runs a set of JavaScripts and test out how fast your browser is.  I tried it on my iMac, 1.83 Intel Core Duo with 1.5 GB 667 MHz DDR2 SDRAM, and found Firefox 3 beta 5 to be the fastest browser.  Oddly enough, Firefox 2 was the slowest, even behind SeaMonkey!

Granted, this is no professional test.  Firefox 2 has the  most extensions on my computer and that could cause slowness.  There are always a lot of factors to take into consideration, but this is what my tests show.