Folks I've been banging my head over a problem for months with Motion getting slow, sluggish, unresponsive, etc. Parameter values would flicker, I could hardly operate (move layers, select objects, etc) without clicking to the finder and coming back into Motion. I've done reinstalls, permission repairs, preference trashing, help from Mark Spencer and other recommendations.
IN THE END it was my 3DConenexion SpaceNavigator
I was thinking too hard on the issue. The consistent anomaly through all of this was that parameters would flicker, even mousing over parameters caused keyframe icons to flicker in the same direction that i moved the mousing arrow (i.e vertically through parameters). Also, undo wasn't working. All this finally made me realize that (the repeated flickering) there was some kind of input device sending commands to the app that was effectively working against my pen on my WACOM tablet or three-button mouse.
I had the SpaceNavigator connected into one of my HP LP2475W monitor's port (which in turn goes to my Mac Pro. I unplugged the space navigator and Motion works butter smooth.
Now the question is - is it really the space navigator (driver) or is it the HP usb hub causing problems.
I wonder if Motion has bad reactions to other input devices.
Hopes this helps someone besides myself. I use my SpaceNavigator for Blender work and occasionally Motion, but less so for the latter.