I once had an identical problem with Windows and the issue was with the graphics card driver. To speed everything up, by default Photoshop transfers as much of the processing as it can from the computer's processor to the graphics card. Some graphics cards, however, need updated drivers to handle that extra work.
With Windows, you just go to the graphics card maker's website and download an updated driver. I don't know how you do that on a Mac, but that's the first thing I would try. Maybe there's a system control panel or something?
Your other option is to minimize the extra work Photoshop gives your graphics card. This is really easy to do. From the Edit menu, select Preferences, then Performance. You'll see a box for Graphics Processor Settings.
I would click Advanced Settings and experiment. Try changing the Drawing Mode from Advanced to Basic. If that doesn't work, try unchecking Use Graphics Process to Accelerate Computation.
If neither of those work, uncheck Use Graphics Processor on that main performance screen. Photopshop will run a bit slower, but the problem should vanish.
