Well, there's a simple tip that might help.
It seems Motion's RT performance gets better in some cases when it doesn't have to show the selected object's control elements (handles, lines, paths, etc). You can accomplish this by deselecting the object (Command - Shift - A) or temporarily turning off overlays completely (Command - Slash).
Adolfo Rozenfeld
Buenos Aires - Argentina
www.adolforozenfeld.com