I don't even see my mouse leave my screen, even when I use ground-target AOE on all 5 windows. In exceedingly heavy combat, I might see my cursor flicker, but it's still instantaneous.

You can also use mock mouseover where you don't actually send your mouse to the other window. You simply set up a region where moving or clicking your mouse at X,Y sends a keypress only to the slave and the keypress triggers a hardcoded macro. That is what I use for dealing with procs.