There could be something else at play here that I wouldn't know about, but if there's not; that's windowed mode for you and there's nothing to be done about it unfortunately.
Windowed Mode in WoW is known to destroy fps, some say it's a minor difference and others, like me, claim it's huge. Either it's different for us all or, and more likely so, some people are actually fine with playing the game below 50-60fps.