I'm assuming you have the latest Starcraft 2 update and there was also an Apple update that dealt with Starcraft also. If both are true then you should contact Blizzard's support. I do notice that SC2's animation can get choppy at times for simply things like the opening animation as the menu screen comes up so it could be a software bug.

It happens randomly for me but most of the time it runs pretty smooth. My Mac can't handle the highest settings though. I have it set to medium but my Mac is slower than yours.