The latest version of LR CC 2015.5 still slows down while rendering D810 nefs. I thought this had been cured a few versions ago, but while rendering a few thousand yesterday, I noticed that it was getting slower and slower with time. It started off at about 4-5 secs per image, but eventually ended up at about 12 secs per image. It can be cured by closing LR down and restarting it. Then the rendering goes back to 4-5 secs per image.
Win10x64 on fast desktop 6/12 core with ssds and hdds.