I can throw in another data point. I have essentially the same configuration: MacPro 6 core, 1Tb SSD, 64Gb, AMD D700 Graphics. I have 2 monitors connected. An Apple Thunderbolt Display (2560x1440) and a Dell 5K display (5120x2880). Both displays are set with resolution of "Default for the display", not scaled. Images and catalog on the internal SSD.
Until recently, I was happy with the performance of this system. After the recent updates to LR Classic CC 8.0 and MacOS Mojave, I am having the same problems. I don't know which update led to the performance issues.
Based on the feedback above about the display drivers, I did a quick test on my system. If I run Lightroom on the Apple Thunderbolt Display, I can hit "F" to view images full screen and switch between them using the cursor keys, and the images switch essentially instantly. This is with 20Mb Raw files with 1-1 previews.
If I drag Lightroom over to the 5K display, hit "F" and try to page through the same images, the performance drops to about 15 seconds to display each image. This is using the 1001, 1002, 1003, counting method so not scientific. But it is not useable for reviewing the images. Same thing for looking at images in Loupe mode, although they are slightly faster at about 10-12 seconds. Probably because they don't use quite as much screen space in that mode. Develop goes to about 18 seconds to switch an image. Basically, Lightroom is not useable on the 5K display.
Very disappointing.