StarCraft II on Radeon X1600

So my Core Duo iMac runs StarCraft II notably smoother when I'm in Windows XP as opposed to OS X. When the graphics are set to the minimum settings, it plays fine in Windows XP, but OS X frequently slows down and tells me to close other applications (I have no other applications open). It would seem that Blizzard didn't get a chance to finish the OS X version. I would actually say that it's unplayable with the X1600 in OS X.