Ray tracing is embarrassingly parallel, but it requires having most if not all of the scene in memory. If you have X,000 machines and X,000 frames to render in a day, it almost certainly makes sense to pin each render to a single machine to avoid having to do a ton of moving data around the network and in and out of memory on a bunch of machines. In which case the actual wall-clock time to render a frame on a single machine that is devoted to the render becomes the number to care about and to talk about.
