← Console variables < maxFPSBk
This cvar sets the maximum FPS of a wow window while its not in focus (minimized or playing in windowed mode, and have something running on top of it).
This cvar defaults to 30, however it can be changed to get better computer performance while multi-tasking.
/console maxfpsbk 0
will set the max FPS to 0 when wow isn't in focus.