Some belated thoughts: Please check using eg Task Manager that your CPU renders are actually using all the CPU core threads they should.

As I mentioned above and YPhil & Co seriously extended, CPU and GPU renders may need very different 'Vols & Buckets' settings. For Superfly, Progressive on this now-ageing twin-GPU system, I set 1024 unless Poser crashes / renders blank, when binary chop until happy. For CPU-only here or on network-render 'Box', 64 of each seems optimal, as more disables use of multiple CPU threads...

FWIW, I like 'Progressive' as I can tell within seconds if render will look good. Or, yet again, yet again, 'Needs More Light'...