I can't really  help you on shutter settings (I think it has to do with how blurred things are if you're using motion blur)..

The f-stop settings on a real camera are tied to the shutter speed to get a correct exposure.  A lower (numeric) f-stop lets more light in in a given amount of time, so if you want to "freeze" motion, you can use a faster shutter speed and a lower f-stop, and still get a properly exposed picture.

A side effect of the f-stop setting had to do with "depth of field", and that's really what it affects in Poser.  A lower f-stop has a narrower range where things are sharply focused.

Suppose you're doing the standard NVIATWAS render.  You can set the focus distance to be the exact distance from the camera to Vicky, and use a low f-stop (say, 2.8).  Vicky and her sword will be in focus, while the temple will be blurry.  If you use a high f-stop (say, 32), the focus on the pieces in the temple might be a little "soft", but things will be much sharper than in the first example.

The price you pay for the artsy "low f-stop" look is, as you've discovered, processor time.