We are not Adobe in this forum, we are other Lightroom users. We cannot modify the program or fix bugs. We are here for technical support, we are not here for complaints. You need to address your complaints directly to Adobe.
Although I found turning off GPU auto switching in power settings did help improve the performance of my LR 7.3.1.
Also I got cardGFXstatus running and making sure the Radeon is on at all times.
I believe there could some bugs with High Sierra to.
My MacBook Pro is identical to yours. I'm not seeing those symptoms on the 45 Megapixel files of the Nikon D850. I also have a 27 inch external monitor attached. When I am displaying the second monitor function of LR, on that monitor, the image in full screen mode lags the main screen by about a second.