Only other thing I can think of is setting affinity of each window manually; I put my main and 2nd on core 0 + core 1, and the other 3 on cores 2 + 3; otherwise, I don't understand why your fps would be where it is then...something else has to be eating processing power. Not something I'm qualified to diagnose nor able to without being at the computer either lol

Edit: Other than the other obvious bit about turning down settings/resolution on yer alts...I neglected to mention I run my WoWs out of one dir and they all start up at 1024x768 with 24bit 1x multisample in vid options, I leave the alts at that resolution and maximize my main's window lol