Well, I just made a similar scene to the test scene shown above, and B5 was faster than B4 by a few seconds in OS9. And, it was faster by a few more seconds on OS X. Maybe I'm imagining the slowness... I guess my scenes are just getting more complex to render. Unless there are certain elements/textures/maps that drag down B5. But, the render tests are proof for me. It's mysterious, but I can't complain.