Did a test on my 8-core 2019 iMAC (40GB RAM) and checked how the number of images processed in parallel influenced the total processing time. Used my test set of 60 images (total of 1.3 GB) and got the following results:
1 → 190s
2 → 148s
3 → 142s
4 → 140s
5 → 144s
etc.
12 → 138s
As we can see, overall processing time is almost equal with 3 and more images processed in parallel. I did not use the Intel Power Gadget but I suppose that the limit has also to do with the thermal design of the PC, together with diminishing returns of parallel processing.
Note that the test with 12 images processed in parallel was done a few minutes after the test with 6 images while the tests before that were pretty close to each other.
While I did these tests, I discovered a bug that I’ve also reported elsewhere. Setting the number of parallel images, I get 1-2-3-4-5-6-7-7-9-9-11-12. Note: 8 and 10 don’t show and are replaced by 7 and 9 respectively.