I think there is a bug in the GPU support in LR (2015.2.1).
Whenever GPU is enabled, and then in OS X, if you 'swipe' to another space, do something, then swipe back to LR, it will hang. As soon as one disables GPU support, this kind of switching back and forth between Spaces is ok, no LR hanging. but with GPU enabled, it hangs 100% of the time on my iMac 5k with OS X 10.11.
Unfortunately, there are many reported bugs with GPU support. This is primarily a user-to-user forum -- please post a bug report in the official Adobe feedback forum: Recently active topics in Photoshop Family about Photoshop Lightroom.