I'm a Mac user, and I can attest to the widespread complaints on the Blizzard forums over the performance of Diablo III on the Mac, which is choppy, and occasionally even unplayable depending on whether the game feels like it.
It's possible for me to run the game at a perfectly good framerate on maximum settings by rebooting to the Windows partition, but altering the settings while running the Mac version of the game does—for whatever reason—absolutely nothing to improve performance.
Blizzard is apparently aware of these issues, and will be posting a fix shortly. Here's what they had to say:
We’ve been closely monitoring community feedback since the launch of Diablo III. The top issue for our Mac players is performance, especially on low-end cards such as the NVIDIA 9400M.
We’re working on an optimization update to significantly improve performance for the NVIDIA 9400M. Other cards will see a modest but noticeable increase in FPS. This fix will be available in the near future.
Let's hope that "near future" is in the days and weeks and not the months and years.