WoW has terrible framerate issues. One of the neater tricks is to use the launcher. You will want to use the launcher so when launcher.exe is running, bring up the task manager to kill explorer.exe and then start the game. It frees up a significant amount of lag that way, especially if you play the game in full screen. Do note it'd be difficult to do anything else without explorer.exe running (like access your task bar or other programs).
There are other methods I can search around for you as well, in the forums. I have a couple of nerd friends that are experts when it comes to this, so I'll direct them to this post if my solution doesn't help any.