That's really interesting. With my slow machine and XP, the "Create photo" plugin appears to render more slowly than the default renderer with larger buckets. Admittedly, it's been a while since I compared them, and my timing of the default renderer may have been a bit haphazard. Or it may be that in faster or multi-multicore machines, swapping buckets in and out of memory is more of a bottleneck than the actual processing.with the "Create photo with renderer" plugin the rendering uses smaller tiles, this means the processor load does not drop after each completed part of the pictures I have a time of 1:48min
For curiosity's sake, are you running it with or without hyperthreading?