There should only be 1 IBL necessary for any given outdoor scene.  And, at best, should also be lower in intensity, depending on the scene in general.  Lowering your diffuse_value on your PoserSurface for all objects will help alleviate the render load; most things in nature will not reflect diffuse lighting 100%.  Light sets are memory hogs, and the more present, the more likelihood that a render can fail.  However, P9 should be stable enough to handle several lights in a scene.