I loaded Virtual PC 5 for the Mac on an iBook with 384 megabytes of RAM, running OS 10.1, and the performance wasn’t just poor. The computer was unusable.
Connectix admits there’s a serious problem, and says it lies largely in the architecture of OS X. Here’s an discussion thread about the situation, along with this this PDF document explaining the technical issues.
Connectix needs to fix this. Quickly.