solo, its definitely a heck of a lot slower than if you were running it natively, but you have to remember Virtual PC's purpose: testing, not performance. I think a lot of people use it to run other versions of Windows or Linux, especially if they're testing software. If you're seeing if certain programs run without errors on Linux, for instance, speed isn't going to be a major concern. Plus, Virtual PC is fairly stable, which is a much more important feature than speed.
If you're interested, you might want to check out VMWare. I've used it a lot more, and just as a preference, I like it a lot more.