Why? Because Lightroom has to generate a huge number of pixels to display on your dual 5K monitors after you make your edits, and this is taking time. Especially if you have done intensive editing on the photo, using lots of brushing or spot healing, then you are asking your hardware to do a huge number of calculations ...
Have you tried experimenting with turning your GPU acceleration on or off to see if either way is faster? Use Preferences/Performance Tab/"Use Graphics Processor"
Well - on my windows computer I had the same issues and I'd like to think that both apple and adobe are able to make an application with more speed.
The problem exists even with minor changes in the develop module.
Yes - with no luck I have tried changing the GPU-settings in LR......
The Secondary display does not use GPU acceleration. A high resolution secondary display will be much slower in rendering the preview image than the primary display.
I prefer using only one monitor instead of two - and this was actually the reason for me to buy a second monitor :-( I just thought it was my old windows machine that was too old.....