Slow on iMac

[Urkle]

what resolution do you run the game at? and what texture quality is it set at?

My first thought is that the video card doesn't have enough video memory free so it's swapping textures in and out of the card (which will cause the game to be laggy). I know on my home linux box when I upped the system to 4GB of system ram instead of 2 (so that the on board ATI HD3000 would use 512MB instead of 256MB) it made a world of difference.

Do recall that Mac OS X is using quite a bit of that RAM for all the programs running (ie.. to do the expose effects etc.. as that is all run through the card). So 512MB is really helpful when running snow leopard. Plain leopard wasn't as "hogging" of the video ram.