This tute over at DAZ looks good for cutting down rendering times in complicated scenes:

Introduction
A complicated background prop, such as a shelf full of various books, uses a lot of geometry information, as well as texture files. Such a prop adds realism and detail to your renders, but takes up lots of memory, lengthening render time. Using D|S' Export as OBJ and Collect Maps features, you can reduce the impact of complicated props on your system, speeding up your renders. We'll be using a set of full bookshelves in this example, but this also works for dolls, messy desks, garages full of stuff, you name it!

http://www.daz3d.com/i.x/tutorial/tutorial/-/?id=2282