Yeah, me too. At least on a G5. It happens when I get a lot of green Preview showing in the the Timeline, from speed effects, and from moves on large 2k still images. The TL responsiveness bogs down to an intolerably slow pace.
Of course, FCP 4 on a G5 is not yet qualified, and probably won't be until Panther is out. So we'll have to revisit this on a G5 once they ship a version that is qualified.
--Ralph