Performance glitches

when I do batch inversions Lightroom behaves weird.
for the first few negatives I have full CPU utilization but after that it goes down to around 30%.
I checked the LR Buffer size, and its not even half full.

I can see that too, but I never got the impression that things got slower.

The CPU hump happens when the first 10 (of 60) images are analysed by NLP, after that, load decreases. I thought that creating previews might be the cause, but I cannot see any change in the plug-in’s preview folder (on Mac).