Carrying on from my last post I thought it would be a good idea to explain exactly what BSP is. Here is an extract from a blog that is dedicated to decreasing rendering time. The link is here http://renderfaster4maya.wordpress.com/ It has some really useful information that will help a lot of people when rendering complex scenes. The blog also goes through how to tweak the settings to get the best result .... probably should have read this before doing my tests!
What is BSP?
Mental ray uses BSP, or Binary Space Partition, to speed up its raytracing processes. Optimizing BSP settings can improve render speeds.What is BSP Size and Depth?
Mental ray, to run efficiently, needs to know where each polygon is within your scene. BSP organizes your scene into sections. Each section stores a limited number of polygons. This limit is the BSP size.
If a section exceeds the limit with too many polygons, the section splits into two. This way, there will be a smaller number of polygons in each section. The BSP depth limits the number of splits.