To answer your question, yes.  I have had some renders bog my system down so much that it required a reboot to remedy the load.  It is best to eliminate as many props and figures from your scene that is not visible.  Reduce your dependence on too many lights.  For distant objects, use lo-res texturing, no need to see detail in those cases.  And if your actor is clothed, make those body parts that are hidden invisible.  Poser will not render objects it clearly does not see.
About morphed figures, keeping it simple will reduce the file size.  Keep the morphs important to your scene and eliminate the others.  All morphed characters should be saved in your library until needed.  Once introduced and posed, delete the extra baggage.