the FPS is set using the 2 fields under the wow user name. maxfps and maxfpsbk

when you set the maxfpsbk to 5... that means, when that wow is not active (not the one you are using at the time), wow should drop the FPS to 5, radically reducing the amount of work required and dropping the load on the cpu/gpu

once you click on it, the maxfps will be used... telling wow to generate that many FPS while active.

my machine is pretty nice, and i run at 20/42 ... meaning 20 fps for all wows in the background and 42 fps for the one i am on. this usually leaves me plenty of cpu/gpu for the other stuff i have running on my machine.