Sometimes I get an unusually long freeze for a moment after hitting undo. When I move the playhead, a pinwheel cursor appears and it twirls for approx 5-10seconds. Multiple Undos in a row can cause lengthier pinwheels.
on a more recent Macbook Pro and older Mac tower.
any ideas? preferences settings I could adjust?
This could be lots of things like background cacheing, multi processor rendering, complex compositions, 3D or particle effects.... Without a complete breakdown of the project that is causing this problem it is very hard to say. If it is happening on every project then I'd look at your available disk space first and cache settings second.
As the others said, you need to provide more info. The only times when I have seen this behavior is with certain "bad" footage types like flakey MPEG files or audio which cause AE to re-decode the entire clip instead of just the visible frame and natzurally this can take time.