Im achieving same issue, both my D810 & D850 files LR was terrible, an editing session that was supposed to take 20 mins took 2 hours its just painful!
I maxed my iMac ram and still no improvement, not sure if you are using iMac but this is what response I got from someone who has been helping me with my issue.
I think you overestimate how many 5k iMac owners there are that process high megapixel raw images in Lightroom. The ones that are there definitely have complained here (quite a few actually). That is just some major demand on the machine to push 5120 × 2880 images (i.e. about 14 megapixels) to the screen constantly coming from 45 MP source files. Even from more moderate megapixel raw files, a 5K screen is pushing the boundaries of what is possible even with modern hardware! This is what my test of running at lower resolution was designed to test for. You need machines like the just released iMac Pro or equivalent PC hardware to push that many pixels while rendering raw files. What's probably happening with your machine is that thermal hardware protection is kicking in when you're pushing it hard. Lightroom should protect itself a bit better against that though and not allow itself to get to that state that easily.
Has yours improved at all?
Nope, still absolute garbage on BOTH the higher powered, SSD, fast HDD desktop AND the more normal laptop. I'm STILL running 7.1 on both 'cause 7.2 is UNUSABLE. 7.0 was unusable. 7.1 is pretty good so it's NOT a resource problem or an "underpowered" system. 7.2 is garbage AGAIN.
This ISN'T an iMac, and I'm NOT running 4K or 4K monitors.
You may have noticed that Adobe is VERY good at incessantly touting their "performance improvement" but less so with successfully resolving the problems that many/some/a few (it doesn't matter how many) have. And, of course, the original entry has been sitting here like feces in a punch bowl for 3 weeks.