Power consumption is super-linear with clock speed. This means that in general splitting work from one core on to two cores running at half the speed will save

Power consumption is super-linear with clock speed. This means that in general splitting work from one core on to two cores running at half the speed will save power. I would expect that in the long term, GPU rendering would be more efficient. In the short term the fact that the CPU is having to do a lot of work to manage the GPU may make it less efficient.