i have seen a situation where the wow window won't realize it doesn't have focus, and will try to remain at maxfps (30 fps). at which point, 2 or more wows would be trying to get 30 fps, instead of 1 doing 30 fps and the others doing 12.

i normally run with the fps shown (ctrl+R to toggle it on by default)... and if i notice one of the windows above the maxfpsbk, i mouse in and the mouse out. nothing more. (with focus-follows-mouse enabled). this usually alerts the wow window of the fact that it lost focus and it drops its fps from maxfps to maxfpsbk