Well, it's kind of a Mac problem and a software problem.
Apple changed an API in El Capitan that caused an issue with the brand-new preview architecture in CC 2015. This is explained in Todd's post here: Re: After Effects 2015 won't preview in real time after El Capitan update.
As he says, the next update to AE should fix it, but there are two decent workarounds in the meantime.
The first is to hide the preview time indicator while a preview is running as is mentioned in that thread. It's really simple to do. Just press the ` (grave) button when your mouse is in the composition panel to make it full screen - thus covering the timeline panel. (It's the same key as the ~ [tilde] key on US English keyboards.)
The second is to use CC 2014. There are a few things missing in CC 2015 that you might want in CC 2014 anyway (see this page: features not available in After Effects CC 2015 ).
AE CC 2015 is the first step in a major re-architecture, separating the renderer from the UI for the first time in AE's 20+ year history. This is a big job and it's clearly still in progress. It's a necessary
step for improving After Effects performance, but it's taking a while (and multiple versions) for them to get through it. So, during this time of transition as they're introducing elements of the new
architecture, sometimes it may be easier to use the older version to get a job done.
Also, double-check that your AE is updated to verson 13.7.x
Thanks a bunch, Szalam! I guess I wasn't asking the right question when searching the forum. It looks like using CC 2014 would work best for me now.
You've got to use what works for you!
For me, I like working with the snappier interface in CC 2015, so I'd deal with having to do the workaround to make previews realtime and then I'd open my project in CC 2014 to render with multiprocessing.