According to benchmark tests conducted by Microsoft, IE8 keeps pace with Chrome and beats Firefox. In real life, though, unless you have a pathetically slow connection, load-time comparisons make little difference.
And if you have a slow, old computer, then neither of these three browsers would work well. Opera outperforms them all. So, unless sparing 20 megabytes is really an issue for you (if it is, buy a hard-disk, they’re not that expensive) give this a whirl, and if you are using older versions of IE, switch to IE8 fast for security reasons above all else.