Some people are stuttering badly in Dalaran for 4.0.1, even though their pc should easily be able to handle it, and they never had lag before 4.0.1.
What I found is, that it only lags while being close to, and more importantly: facing your camera to busy places. My "fix" for this has just been pointing my camera to low visited places or the air.
Most people most likely don't care about this topic but I guess it's a nice handy tip for the ones experiencing lag :3