Mac OS are so cumbersome to program and optimize for, which is why the tech and software world does not pay much attention to the platform. Sure there are some that do but the majority of things that are released, even outside of games, is not meant for Mac. Unless of course apple themselves make them, which even Apple creates for PC because they know that is where the majority of the market is.

Also WoW is a bad way to compare performance, it is 10 years old. With plenty of time to optimize for Mac.