- Safari is really fast at rendering CSS and slow to start up. - Firefox 3 uses the least memory (though this isn't based on a "soak" test. - Opera 9.5 is generally quick but is beaten on CSS and uses a fair bit of RAM. - IE 7 isn't really good at anything except maybe loading time. No surprise on the loading time, but it uses the most RAM for open tabs!
JavaScript performance appears to vary less than some browser developers like to emphasize, IE is less than 3x slower than the fastest browsers in the JS test. (Opera + Safari)
I can anecdotally confirm the Safari start-up times on Windows, although it's a very different story on OS X. It does have to load its own GUI toolkit, although Firefox (XUL) and Opera (some in-house thing so it runs cross-platform and on phones?) also don't use native Win32 widgets.