Wrong. Go to Dalaran with 150 people in the main quad in the evening. On a Dual Geforce 295 liquid cooled quad core gaming tower at 1920x1200 I get 40fps. Anything I do in Crysis, FarCry, Fear2, Call of Duty Black Ops...much higher frame rates, usually over 100-150fps.
The low frame rates are related to the syncing with the server, when you have that many people in the one area, it gets messy (or that what I've heard).