All I did was upgrade to Big Sur, but now Lightroom Classic runs very slow under specific scenarios on my 16 core Mac Pro 2019, 192GB RAM, 2TB SSD with Radeon Pro Vega II 32GB GPU Card. Specifically I noticed two major issues since upgrading to Big Sur:
When in Library module, if I zoom an image to 100%, trying to pan the image is jerky and slow. Panning the same zoomed image in develop module works smoothly as expected. PS: Hardware GPU acceleration is set to auto.
The second issue I have is that within 5 minutes of viewing photos, adding some adjustments like spot removal, my GPU RAM usage jumps from about 2GB to 32GB, which then causes trhe Apple WindowServer process to consume large amount of CPU and my whole machine becomes extremely slow and jerky. Closing Lightroom immediately causes the RAM usage on the GPU card to drop to standard lervels (2-4GB) and the performance returns to normal.
In fact, with Lightroom closed right now my GPU RAM usage is 5.7GB. I launch Lightroom Classic, GPU RAM usage is still 5.7GB. I open the develop module, it jumps to 9.92GB. I then take the heal spot edit tool, use 75, 75, 100 brush and click on a photo (Sony A7R4). GPU RAM usage jum,ps to 12.48GB. Add another spot, it jumps to 14.7GB. Add 17 more spot corrections, and my GPU usage is 100% - all 32 GB of VRAM is used. WindowServer starts running up to 40 - 80% CPU and everything slows down.
Close Lightroom, GPU RAM usage drops to 1.9GB.
This might not sound bad but it means I can edit at most 2 - 3 photos before I have to close and reopen LightRoom.
If LrC 10.0 was working OK and you don't need the new camera support, then I would recommend reverting to 10. This is a relatively straight forward process using the Creative Cloud App (see attached screenshot). Click on the 3 dots button and choose 'Other Versions', then select LrC 10 from list.
Unfortunately the issue is not LrC, at least, LrC before I upgraded to Big Sur was fine, I upgraded to Big Sur but kept the same LrC and the issues began, indicating that it is likely an issue between LrC and Big Sur. This issue began before 10.1 released today, if that was 10.0 then it means this issue I have been experiencing applies to 10.0. 10.1 made no difference.
Library module issues in LrC10 when using Catalina could be averted by using the sRGB ICC profile for your monitor or alternatively calibrate the monitor to sRGB. Unfortunately, Big Sur broke that workaround.
FWIW, I had the same problems as you describe, and above is not only how I worked around it but also one of the two official workarounds. The other being to downgrade to 9.4.
I just tried it. I quit LrC, set both displays to sRGB and launched LrC, but the two issues I mentioned are unchanged. Issue with downgrading to 9.4 is that I have done a lot of work since upgrading to 10.0 and my library will probably not open in 9.4.
I downgraded to 9.4, opened my old catalogue and can confirm that in the Library module, panning at 100% zoom is smooth like before. So then LrC 10.0 is definitely the cause for this issue.
As to the GPU RAM issue, adding 20 spot removal corrections still gobbles up all 32GB of VRAM.
You might wish to add your experience and hardware details to the thread running on Community Feedback forum https://feedback.photoshop.com/conversations/lightroom-classic/lightroom-classic-mac-user-interface-...
PS: Hardware GPU acceleration is set to auto.
So, does LrC report that your system supports full acceleration?
Probably with that GPU
Yes - it says it supports full acceleration.
Radeon Pro Vega II 32GB GPU Card
GPU driver version?
For a MACPro, can you update that driver? Probably from AMD?
/Warning/ Some MAC Pro owners have had issues manually updating the GPU driver if going around Apple for the driver. Get Kernal Panic errors.
I have whatever comes with Big Sur 11.0.1.
EFI Driver Version: 01.01.186
I fear you're being dragged into a pointless Q&A that will lead to nowhere other than even more frustration. When you moved back to 9.4 you effectively removed code introduced in LrC 10 that is the source of your and many others peformance issues. Unfortunatley, the combination of 10.1 and Big Sur make the issue worse rather than better. Adobe engineers know what the problem is, but fixing it in the short term has wider implications for more customers. I wish it were different, but wishing rarely makes a problem go away.
BTW, I found I had no option other than to back out of Big Sur in order that I could make 10 and 10.1 useable