Hey Tom,
I run Motion on a 2.4 ghz MBP and it works okay, but I also have 4 gigs of RAM, which I think helps. Mostly I use it for time remapping, as Optical flow is just so good, but it also puts huge files in the Motion library, which need to be cleaned out after you render your project.
I must say though, after a considerable retiming project it takes a while to get back to normal. It just kind of pauses to catch up or something.
Have a wonderful day.