I believe it does.
Works fine with my new iMac 27" Retina.
I’m not using the 5k display so can’t comment personally. From my experience on the community forums there had been disappointment with LR5 with time lags (loading) even with Library previews in serving up all those extra pixels to the monitor.
LR6 has introduced GPU acceleration in addition to the CPU. Eric Chan of Adobe claims that those using HiDPI and Retina displays should see the biggest improvement. Since then we haven’t seen anything like the same number of issues being raised. So I guess there have been improvements
Response times will always be affected by other factors, for example if your originals are stored on SSD or external drives or connected by USB or thunderbolt.
Here is one link from Mac Rumours but I suggest you do your own research for real life testing and use Google YouTube etc.
Thanks for your reply!