It comes down to the app. The app emulates a particular type of PC with a specific CPU. So, it's irrelevant how much power the host computer has.Even if you factor in only single CPU use, you'd have expected a 2.5Ghz G5 to make a difference but alas not - I felt my Quad was no faster with VPC than my Powerbook.
Having said that, Win 2000 and below, I've found very acceptable especially under OS9.
Unfortunately, any development on VMs went to PC and Intel Mac once the Intel switch happened. We're stuck with what we have with VPC7.