I've used both Imovie for OSX and OS 9 extensively and I'd have to say that it feels a bit quicker in 9. However you can't really do much else in 9 while doing some of the process intensive stuff in Imovie but you can in X. I usually keep Mail and Omniweb running while I build transitions or export to QT so I can do other things while Imovie does it thing. I'm afraid to do that in 9 as apps like Imovie want to hog all the resources.
So to sum up - it's better on X. This will only become more apparent as apps and the OS itself becomes more refined. I can't really stand 9 anymore.