Well, I did some more testing last night. Fairly obviously, RAW file size should be a factor in all this. The only alternative RAWs I have are very small, from my old Panasonic LC1 at 5MP each. So I resurrected a few of those, picked one at random, loaded it into LR3 and did all the stuff I normally do - perspective correction, rotate and crop, basic edits and levels, grads, local brush adjustments, spot removal. And LR3 ran as smooth as silk throughout, no slow-downs, hangs or glitches at all. Every action was real-time and instantaneous. I kept an eye on memory usage, and it never got above 2GB.
So this might explain why some users are perfectly happy with LR3 and others are tearing their hair out. Probably, for every particular PC/Mac hardware setup, there is a RAW file size above which you will experience problems, and below which you will be absolutely fine. This RAW size hinge point might be 12MP on one machine, and 20MP on another, depending on your hardware. So some users on some machines with relatively small files will see no issues at all.
With LR 2.6 and 2.7 even my 39MP files ran just fine. On the same PC, LR3 is pretty much unusable if I activate the Lens Correction panel - even if I don't, multiple brush edits will slow it down very noticeably, which did not happen with 2.x