Slowly getting there .... It seems that there is a conflict  in Poser 13.1 up, I was able to speed up the setting or slow them down

Here is a sample of the setup used that took forever after the update of Poser 13.1 up ( Still does ) but It can be fixed without support, I might upload a Pz3 Fix fog file when getting a good speed balance.

This is the latest setup I tested with the fix. With the Box Fog

 

Using the sample that , I think, hborre shared at the time .... a good option of many that are giving good results with the 3D Cloud tex node

This is the final render Optix High 1500x2000 ++ rendered in 1min 37sec