You mentioned that your cpu was at 100%, so you may have already done this.

Open your WTF/Config.wtf file in each of your WoW directories with a text editor. Find the line that says SET processAffinityMask "X" - make sure X is 15. Additionally, make sure no other applications are controlling which cores WoW uses. This will ensure that each copy has full access to all 4 cores (even though they will only ever use 2 at once).

Again, you're probably already doing this or something similar. However, if not, this will give you a 30-40% increase in FPS.