Issue: Playback of cached previews is not real-time for compositions on installations of After Effects on Mac OS X 10.11.
See this article for a description and explanation about the issue: known issues in After Effects CC 2015 | After Effects region of interest, see the section entitled, "playback of cached previews is not real-time on Mac OS X 10.11"
You can work around the problem by hiding the preview time indicator (red line that moves across the Timeline panel during previews) during previews, such as the following:
Status: The fix for this issue is targeted at our next update. You can also file a bug report here.