I use Poser 7, not pro. But I DO have a significant push in speed when I render with more than one thread - if the render takes long enough.

If you use multiple threads, the overhead is larger. So if you have a scene which is rendered in seconds, you won't see an advantage in using multiple threads, because starting the render already can take minutes.

If you have a scene with reflections, refractions and/or AO, you will see a difference. If your scenes are big and/or complicated enough for hours of rendertime, you WILL see that rendering in four threads brings an advantage.

Compare it with delivering a sixpack - if you just have to walk for a minute, you are faster without using a car, because starting it and finding a place to park takes more time than just walk over. If you have to deliver it some hundreds of miles away, the car is much faster.

The job defines the tool.

A ship in port is safe; but that is not what ships are built for.
Sail out to sea and do new things.
-"Amazing Grace" Hopper

Avatar image of me done by Chidori.