[Josh Evans]"Is this going to slow things down at all? "
Not necessarily. If you have two separate display cards (or have more VRAM on one than Motion can use effectively) spanning two or even three displays won't affect your operation.
If you've just got the one card, or it's a bit underpowered, I wouldn't take our word for any of this. Conduct your own trials to see if moving everything to a single window of managed panes affects your performance. You'd be doing us all a great service.