If both monitors are running off of the same video card, then it shouldn't matter at all (although you can give it a shot to see if anything changes).

Depending on which software solution you're using, you may be able to improve performance by choosing to have the slave windows render at a lower resolution than the main. Doing so would consequently distort your mouse broadcasting, but again, depending on the software you're using you may be able to "scale" the broadcasting to "fit".