Using MaxToArnold 2.3.33.2019 and backburner. 16 cores is only twice as fast as 6 cores and 24 cores seems to be the same render times. I can see 24 buckets, but the time stays the same. Any suggestions?
Can you monitor the load on the threads?
Try setting the threads manually. Should you not see 2 threads per core?
Also Windows scheduler is a issue with a lot of cores, there is software for a work around.
Please post Arnold logs for both renders.
If you have a 12 physical core machine, then with hyper threading (HT) you have 24 logical cores. HT usually only gives a small improvement of about 10-20%. That means going from 6->12 physical cores should be about 2x faster because these are real cores being used. Past 12 threads you're not going to get much of a speedup which explains why 16 is still only twice as fast as 6 and 24 is about the same as 16.