I know Chris Georgene has been putting CS4 through the works as a beta tester, and is writing a book on animating in Flash CS4. He uses the PC version, and hasn't mentioned anything about it running slow.

CS4 is probably a bit slower than CS3 on the PC, but not as slow as you're describing. It wouldn't be the first time Adobe has delivered an inferior product for Mac users.

I'm waiting for the demos to come out, and will test it on the PC and Mac (on the same system) and find out which runs smoother.