Right, it will be rendering the same resolution as the main window. If you would rather not have this slight performance hit (it depends on the hardware vs the number of game windows) you can instead disable window switching. To see the difference in performance for your particular configuration, you would really just have to try it out -- and of course there is a free 7 day trial.

Either way, general performance-wise you are best served by having video options in WoW set to minimal, and the camera angle in the game pointing straight down at the ground on your background windows. The camera angle thing also makes it much more convenient to do targeted AoEs. I should do more videos