I'm not sure if this is a Lightroom problem or an f.lux problem, so I'm posting in both places. F.lux is software that automatically cuts out blue light at night so your screen doesn't disturb your sleep cycles as much, but you can set it to shut of for particular software. Here's the bug report I posted at f.lux forums:
I installed f.lux a week ago on a 2012 MacBook Pro Retina running OS X 10.11 El Capitan. When Lightroom was in the background, it would become unresponsive (beachball only, OS X reported it as not responding). I uninstalled f.lux and the problem stopped.
I had disabled f.lux for Lightroom (latest CC release, mostly the same as 6.3) so I could get accurate colors for photo editing, which may or may not have been related to the problem. I forgot to check if reversing f.lux's Lightroom exception prevented LR from hanging before I uninstalled.