There is a bug in CC 2015 on El Capitan. I'm surprised you haven't seen the discussion about it as there have been many threads! (It's also mentioned in a few official Adobe blog posts.)
There are some workarounds.
One is to dock another panel below your timeline panel like so:
Note how the render queue is docked underneath the timeline panel. You could use any other panel you would like; this is just an example.
A second workaround is to hide your preview time indicator. You can do that pretty easily by putting your mouse over your composition panel and hitting ` (same key as ~ on US English keyboards) to make your comp panel maximize and cover the timeline panel. (This doesn't work if you've got your comp panel on a different monitor.)
Anyway, if you want an explanation of the issue (and a very long conversation about it), see Todd's post this thread.
Alternatively, you can apply to help test the After Effects CC 2015 (13.7.1) patch update which fixes that bug.
I'd seen lots of threads about there being an issue with El Crapistan, but hadn't seen that solution mentioned.
Thanks for pointing it out though. As weird a work around as it is, I'm happy to get my speed back.
Glad to help!
Hopefully the patch comes out soon, but docking a panel under there is a pretty painless workaround until then.