I haven't done side-by-side comparison, but I have tried various versions VMWare, Parallels, and VirtualBox on Mac OS X with Windows guests. To me, VirtualBox seemed to be the fastest, with VMWare and Parallels about the same, lagging a bit behind. However, Parallels 4.0 claims to be 50% faster than previous versions, so I'd suggest taking a look at that if you haven't already.