You only look at one WoW window at any given moment and it's usually the one where your mouse is. If you cap all your backgrounded clients to 15fps, it'll give your foreground client the lions share of the GPU and you'll get a nice high frame rate in the window you're looking at.

As soon as you move your mouse to another window (assuming focus-follows-mouse is enabled), that'll become foregrounded and it's framerate will increase, while the previous window will drop.