I have found that imported models, when used in large, or even medium amounts can dramatically increase render times on any machine. Sometimes if there are enough polygons in the scene the render will appear to stop totally, usually on the last pass, sometimes halfway through. I have made scenes with around 200-300 groups (thousands of Bryce primatives) with full multi-channeled textures, rendered at large sizes, that have completed within an hour...The last experiment I did with imported trees/plants (from the Bryce2 CD ) I abandoned after duplicating the trees/plants few times my computer turned into a slug...