Apparently the Poser programmers have tried to be clever and reserved 1 thread for the fore-ground Poser when running the renderer in a separate process.

Some people have reported problems with running the renderer as a separate process, maybe these people had set the number of threads to 1 and thus confused the renderer.

The manual is of course clear as mud on this issue, it simply states that number of threads is at maximum 4 and should always be set to 4 regardless of the number of cores. This might not be the best advise when running the renderer as a separate process. If you want to do anything else on your computer you should set the number of threads to the number of actual cores (this will leave one core available for other tasks). It also means that running as a separate process on a 4 core machine can never use all 4 cores for rendering.

As usual the Poser developers have chosen their own idea of thread handling over the usual standards.