macOS – Fix iTunes 11 Stuttering During Playback

This might come a little bit as an obvious question but… have you restarted your box since you installed that? A zombie process could be causing delays in other areas.

Check with /Applications/Utilities/Activity Monitor.app and sort by CPU usage to see if there’s anything “red” (zombie) or any process not behaving.

In any case, a sane box restart (if you haven’t already) is usually a good advice. Turn the box off for a few secs and then turn it on again, the VRAM might be corrupted and a simple “restart” won’t always do the right thing. (It happens to me in the Mac Pro).

The reason for mentioning this is that your computer is more than capable to play videos.

Do you have Perian or any other third party codecs laying around that might interfere?

Since more than one application is having issues, it's unlikely to be two bugs and more likely to be a system or hardware issue.

Before spending a lot of time, make a new user and see if that user has issues as well. You could have bad luck and lots of corruption in your user folder and this will let you know if the problem is with the apps/system/hardware or not.

Drive Genius might help to run more involved tests, but it might be cheaper to just buy a spare external drive and back things up. You can zero the drive and clear all bad blocks - give that hardware the best chance to work again. You will know soon after restoring your data on the newly erased and installed mac if the problem is resolved.

Good Luck!