I have the beta installed on my iMac 350 with 196mg RAM and it runs as slow as my SoftWindows applications (about half the speed of the native software). I have also installed the beta on my wife's iMac 350 with only 64mg RAM and it runs the same. No speed decrease, no difference. Apple recommends at least 128mg RAM, but the fact is, it runs slow and jittery no matter what.
I also have seen no difference in speed or performance whether the beta is installed over OS 9, by itself, on a separate partition, on the main partition, using the HFS+ file structure, or using the UNIX file structure. It's just slow.