I scarcely touch iMovie, but my daughter does, so I installed iMovie 3 to see how she likes it come the weekend.
It was
so darned slow at first that it quite worried me! Repairing permissions helped on my G4 eMac 700 mHz (768 MB RAM).
Looking further, I found a raft of issues with an apparently rather buggy release discussed in several places, including
MacInTouch.
We'll see what my daughter makes of it, but if anybody's having bad slowdowns, one fellow on that page I've linked to says he's put iMovie 3 and iMovie 2.1.2 on the same machine.
I can confirm his find that the two apps appear to have their own sets of preference files.