P: Slow UI when using Mac and Custom Display Profile

I have exactly the same configuration and exactly the same problems.  Tried all the usual tricks.   Applying presets in bulk, which took 10-20secs in 9.4 now takes 10-20 _minutes_, and then Lr insists on rewriting all XMP data.  Also, Delete Key no longer removes a gradient, and when removing using menu, the animation is gone.  Seems to be some other weird UI things happening, its quite disconcerting.

I can't recall ever having performance issues with Lr before....

I'm in the same boat.
Mac Pro 2010, 2x3.46GHz, 28GB RAM, 1TB Evo SSD, Radeon RX580 8GB

I've done all the usual checks and tweaks like turn off sync, address, face, turn off/on GPU acceleration, purge cache, build 1:1 previews, delete previews, then rebuild them again.
The only thing that speeds it up is if I resize the Lightroom window to less than half of my screen (4K 27") which is not ideal.

Recent Nvidia Quadro and GeForce cards should be fine but there is always the issue with MacPro compatible drivers.

In your situation, I would contact Nvidia support though they are likely to recommend the latest cards. And check with Adobe support to see what they say about the problems you are having with your card. They might already be planning a fix. That would be free. 

Also measurements I've taken, reported in another thread, indicate that Lightroom is making much heavier use of the processor to provide performance improvements. All develop tools that I've tested are now instantaneous on my Windows Xeon 3.6/4.0 GHz machine with most of the improvements coming from heavier use of the processor. 

People who don't know how video cards work think that we would unconditionally benefit from Lightroom increasing its use of the video card. However for many functions, a video card is a bottle neck. This is especially true if the whole image won't fit on the card. Just take one unrealistic example, if Nvidia were to release a 32GB version of the P4000 and Lightroom were to require that card; we would see a speedup for most functions. But not all. 

Of course, the most straightforward solution for you is to switch to Windows, notice the flood of problems primarily by Mac users. Think what it is going to be like with a new version of Lightroom running on the totally new version of MacOS on a new hardware architecture. This forum is going to be buried with bug reports. 

I know $$$ laptops has similar performance like $ desktop. But this is a 4000$ notebook that could handle the software very nice till today when I updated it. If I was using some crap 500$ notebook I will understand it but this is a "pro" machine and if it cant handle 24mpx raw files there is some problem. And in my experience, the problem is in the software. Or we are now in a time where high-end machines are usable only for one year and then they are good only for youtube?! I also work in Premiere Pro and Davinci Resolve and I learn something there. When you have two software that can do the same thing and one can do it a lot faster than the other on the same machine you can tell that the problem isn't in hardware but in software optimization. I am downgrading LRC and wait for Adobe to FIX this.

Same; mac mini 2020, i7, 32gb mem.

Also I see that all previews are been regenerated, which does not help either. Also opening Metadata is dead slow.

Bill you say Adobe doesn't list compatible cards.  If I was to add an external GPU to my iMac Pro how would I find out which model would be compatible with Lightroom?

$ for $ laptops are much slower than desktops. Nvidia cards are faster than AMD. As hardware gets pushed by software for better performance, the fast equipment gets faster and the slow equipment... Well you get the picture. 

Your MBP is great for Linux software development but for high performance photo or video editing, get a $4-6,000 gaming laptop to compete with a $2-3,000 desktop. 

Yet here we are after spending $6000+ on computers less than 3 years old, our work is coming to a stand still. Even though Apple says they are supported and Adobe says GPU with Metal Support work. And even if they don't support, why is it slower than it was in 9.4?

Still requires a video card supported by Lightroom. Unfortunately Adobe has stopped listing supported video cards so for other than mid to high end Nvidia cards, it's a guessing game. The Vega 56 is a "maybe" card.

Business reality: if 1 in 10 thousand people with a computer do photo editing and maybe 1 in a thousand of them do it on a Mac with an AMD video card... You get the picture. 

Based on your experience, a change in Lightroom v10 probably pushed your card from the "maybe" category into the "unsupported" group. Adobe delivered a significant number of significant performance improvements with V10. It is likely that those improvements will be a setback for some hardware. AMD and Nvidia use significantly different APIs for communication with the card. Add in the variations from one card architecture to another within a brand and the problem becomes obvious. 

Same here. 16" Macbook Pro, 6core, 32GB ram, 5500M 8GB, and after update to LRC V10 it is unusable! With last version it was ok, not blazing fast but 100% usable for work. Now it is working approx 5 times slower. Changing modules, copy and paste photo settings. I am also working on desktop with Ryzen 3900X, 64GB ram, RTX2070Super and after update the performance is same or maybe even little bit faster. So I updated also on my MBP and its disaster 😞