Oddly, this worked for me; and it seems to have worked for others:
WoW-Europe.com Forums -> Performance BOOST ! check THIS out !
1) Open WoW
(title screen, not launcher)
2) Close WoW
3) Open Windows Media Player
(do not play any music)
4) Open WoW (Fullscreen [not window /maximize] mode)
That's it, it got me a 15 FPS increase and a 25 decrease in latency (originally 50).