High CPU usage in WoW, what to do?

Obviously, first steps are to turn off everything unnecessary and see what the behavior is like. It may still be awful, but its a point of comparison.

After that, have you patched recently?

Also, are you getting bad FPS behavior in addition to high CPU usage? If so, is it uniformly bad FPS behavior, or are you having spikes in FPS? Both are known problems. Unfortunately, there have not been any good answers produced by the community or Bliz and there are active (bumped) threads in Wow Tech Support, where you should probably spend some time searching anyhow.

You may want to try moving your WTF, WDB and Interface folders to your desktop, restarting WoW, and checking how it works without any of your add-ons. They can be moved back if its not the issue.

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No consolation, but you're way better off then some, with frames dropping from 60 to 5. I was talking about shutting off full on programs like AVG and ZoneAlarm. Virus scans in particular are notorious in this respect.

Then, I will point you over towards the 3.1 performance optimization thread, which details a huge list of potential ways you can shift your performance. As far as I can see, there doesn't appear to be a magic bullet I can provide, as the causes are varied, and you'll need to do some methodical checking on your own to sort out the issue.

There's nowhere I can't reach.