@kurttasche If you have a lot of capacity, you can reach a point where your CPU becomes the bottleneck, at this point it is highly recommended using GPU assisted mining.
The screen below is from times where CPU miner did not have AVX/SSE support, but it illustrates just perfect what the benefit of using GPU is ... same plotsize (~80TB), first two blocks mined with CPU and after that 2 blocks mined with GPU.
For the second 2 blocks the GPU has a lot of load, that is not shown here. As far es i remember it was a 280x @ 50%.
More details here: https://bitcointalk.org/index.php?topic=1170987.msg12329106#msg12329106