Fresh install, eh? Could you fly to an empty zone (Barrens or Westfall) and check whether it lags? If it does, could you go afk for 20 mins while your character is flying/doing nothing (do not log out) and after you come back, check whether it is still lagging?
I had not thought of that. I will try that tonight. I am also going to do a fresh Windows 7 install. I am thinking DX 11 might be fubared for some reason.