Ok, I made several tests, raising the bucket size first to 64 then to 128. I couldn't see any difference in the render times or in the processor/memory usage. I then tried increasing the render size from 320x240 to 640x480, keeping the bucket size at 128. As hard as it was for me to believe, the render time was still almost exactly the same, but the processor usage increased to about 60% and the memory usage went up by maybe 30Mb. Evidently this particular animation is so trivial that it's hitting some kind of fixed overhead of the renderer that has a much greater influence than the actual computation involved. I could probably render the same number of empty frames in essentially the same length of time. Gromit