You could try using the presets to see if anything provides what you want.  There have been changes to the Adaptive Sampling which seems to allow a drop in the pixel samples and thus a drop in render time.  I have so far used the GPU high preset which has Adaptive Sampling at 0.001 where I was using 0.002 in Poser 12 but the samples have dropped to 25 in Poser 13 where Poser 12 was set at 50.  The quality is about the same as what I was getting with the Ultra GPU preset in Poser 12 and the majority of my renders were much faster but it did vary a lot from scene to scene.

When using the CPU there was no sign it was using all the cores but checking Windows resources all the threads were maxed out, at least on my machine.

 

 

I use Poser 13 on Windows 11 - For Scene set up I use a Geekcom A5 -  Ryzen 9 5900HX, with 64 gig ram and 3 TB  storage, mini PC with final rendering done on normal sized desktop using an AMD Ryzen Threadipper 1950X CPU, Corsair Hydro H100i CPU cooler, 3XS EVGA GTX 1080i SC with 11g Ram, 4 X 16gig Corsair DDR4 Ram and a Corsair RM 100 PSU .   The desktop is in a remote location with rendering done via Queue Manager which gives me a clearer desktop and quieter computer room.