One thing that helped me a little in this situation was to set the processor affinity: Bring up the windows task manager, switch to the process tab and right click each instance of wow and make sure all 4 processors are checked. I think some people on these forums have suggested giving each instance of wow its own core for affinity but that would not be a good solution, let the os spread cpu use by checking all four. **you will have to do this each time you run wow

Are you using keyclone to setup your windows? If you are then as a test try just running them in windowed mode normally without keyclones setup to see if its better, if it is you may want to consider wow maximizer for layout (i had this issue)

Another issue I had (while using maximizer i think) is that the client was actually running at a higher resolution 1024x 766 and i was laying it out at 800x600 compressing a higher resolution window thus wasting massive amounts of resources x 4 windows.