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

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.

For curiosity's sake, are you running it with or without hyperthreading?