I only have CPU on my computer. My render times came out as follows:
tiles: 64
time: 28:58.86
tiles: 128
time: 29:44.46
tiles: 256
time: 32:35.72
tiles: 512
time: 51:27.94
I only have CPU on my computer. My render times came out as follows:
tiles: 64
time: 28:58.86
tiles: 128
time: 29:44.46
tiles: 256
time: 32:35.72
tiles: 512
time: 51:27.94
Try 16x16 and 32x32 as well … 
I did go back and try 16x16 and 32x32 with the following results:
tiles: 16
time: 33:33.40
tiles: 32
time: 28:13.85
32 tiles seems to be the tipping point. Faster than the 64x64 but 16x16 took longer.