I haven’t overclocked my CPU yet, so my results probably could be improved. These results are using Blender 2.8.
Tiles Time
256x256 2 minutes 19 seconds (GPUs + CPU)
256x256 2 minutes 5 seconds (CPU only)
128x128 57 seconds (GPUs + CPU)
128x128 1 minute 36 seconds (CPU only)
16x16 14.25 seconds (GPUs + CPU)
16x16 1 minute 10 seconds (CPU only)
8x8 15.48 seconds (GPUs + CPU)
8x8 1 minute 11 seconds (CPU only)