i wouldn't worry too much about that message.
It just means the CPU is running slower, when there is no application demanding more cpu-power. You often see this message in ISBoxer when wow isn't running yet.

I've got the same CPU and its never really maxing out, so that should be sufficient to run 5x wow.

You might want to tune down some video-settings in wow. Try them one at a time and see if behaviour changes. For me for example, turning down shadows from ultra to good improved fps from 30 to 60.