So you CAN hit that same bump with a PC -- when it walks out of RAM and starts spinnng the hard disk. That is an incredible slow-down when it happens. Minor things that can help include deleting parts of the mesh that aren't seen, setting fall-off on lights, reducing the photon count, reducing the bounce number. Soft shadows take time in all applications, and radiosity is, not surprisingly, a hog. I still see no reason I couldn't render that one over lunch...and as I said, my machine is two generations older than yours.