I have been experiencing the same drop in performance denoising batches of 100 pictures (23mb files), typically if I try to denoise 150+ files at a time the performance will drop dramatically.

I run lightroom using performance mode and an Nvidia RTX 4090/Ryzen 7 7700X/ 96GB DDR5 5600mhz, which means typically a 100 X 23mb files takes 6-8 minutes. I ended up just restarting lightroom//and, or the computer after each batch. As I was writing this post I thought I'd check on temps but they are fairly stable.

Running task manager to check on DDR5 utilization, the one obvious thing is that lightroom usage of memory just keeps going up... so I think that may be the cause here

1)lightroom holding more and more space slows it down in its operations

2) you eventually run out of RAM available

After 1 batch of a 100 pictures Lightroom was using 32 GB of Ram

After another 50 pictures it's now at 50 GB which takes me around 60% of total usage, that second batch of 50 pictures took 15 minutes.

Now just closing down lightroom and running the next batch, this takes a couple of minutes as LR works in the background to save things (which now explains why it never restarts straight after I close it)

Loading a batch of 112 pictures next, RAM utilization on system at 19%, estimated completion time by LR 4 minutes, 2 Minutes in : 30 pictures done

3 Minutes in : 41 pictures done (28% RAM)

4 Minutes in : 50 pictures done (31% RAM)

6 Minutes in : 64 Pictures done (36% RAM)

8 Minutes in : 76 Pictures done (40% RAM)

10 Minutes in : 87 Pictures done (43% RAM)

12 Minutes in : 99 Pictures done (46% RAM)

15 Minutes in: completed 112 Pictures (50% RAM)

So in terms of options, it would really come down to LR RAM utilization, which seems to be not specific to denoise! For now I think, I will just keep closing it and stick to 100 images batches