Reading your original post, it appears that your issue is due to your desire to use a display scale that is not “best for display” because you want larger text. For support on this you should check or contact EIZO Or simply use the default “best for display” resolution.
I figured this out already.. It seems that the MacPro 2013 does not have enough GPU power to smoothly run a 31 inch 4K monitor with scaled resolution.
That's why I use the screen now in "best for display" which is the native resolution without scaling.
But my Lightroom issue did not disappear. In fact, Lightroom runs slow on all my 3 screen sizes (31, 27 and 24, where only the 31 is a 4k) in single monitore mode. Which means, the source can not be the display.
The slow performance in full screen and the overview mode of the library started after migrating the system from one MacPro to another one, using the OSX migration assistent.
Already deleted LR cache and preferences, which did not solve the problem.
Tried 3 different screen sizes, did not solve the problem.
Used dual and single monitor setups in all kind of combinations, did not solve the problem.
Apple hardware test does not show any hardware issue
Black Magic Disk Speedtest shows really high dis speed
CPU/GPU load statistic does not show high load, while using Lightroom
Tried LR 7.3, 7.4 and 7.5 without a significant difference
Now I'm shortly before a complete reinstallation of my whole system with a manual migration of the data to get a clean start. I'm just scared of the time needed to do this. :-/