I just tried the game on Win7 via Bootcamp on the same hardware and there were no performance problems. (well, since i use apples touch mouse the game constantly registered me as scrolling which was kinda annoying in battle - would be nice if i could turn that of in the mac version or reduce the sensibility)
When i start the mac version everything lags, even the world map or the start menu. I would not go as far as saing the PC Version is flawless in speed, but the lag i had over there was something i would have not noticed at all since i am fairly used to everything working a bid slower on this old computer. But hey, maybe there finaly will be new iMac next week.