I have been getting serious framerate issues ever since 5.1 came out (yes, I've been tolerating it for quite a while now). I have an iMac. My frame rate when I'm instanced (e.g. raids, BGs, scenarios) is generally okay (around 30-40 fps), but as soon as I am in the "real world" (e.g. Pandaria, Orgrimmar, etc), my fps drops to 2-3. It is most noticeable in high-traffic areas, like Org or Shrine of Two Moons. Prior to 5.1, I had about 30-40 fps everywhere (both instances and "real world").
I have turned almost all my settings to the lowest. This issue seems to happen even when I disable all addons. Any ideas or suggestions as to what is happening? Was there a major graphics changes that affected Mac OS X in 5.1?