Quote Originally Posted by 'cryinsham',index.php?page=Thread&postID=211381#po st211381
try hooking both monitors to the same(primary) card, frame rate aare dropping because windows is software renderign from your second card eating your precious cpu cycles.
This in one hand. On the other hand a dual core will never be as adequate to run 5 instances of WoW as quad core CPU. The pain will become more obvious once you move into outlands and northerend.

In the meantime here are some other things to try and and check:

Try running two instances of WoW, and note the results. Then three, then four. Try this with just one card running, as suggested above, check your CPU performance in task manager. If your framerate decreases as your number of instances increase, then the CPU is maxing out.
Additionally, and if this started happening recently check (again use task manager) if there is perhaps a process easting up your CPU cycles.