CPU: Intel i5 3.4 GHZ
GPU: nVidia GTX 980
My fastest time using GPU: 480x270 tiles was 1 min 11 sec
I bumped it up to 100% resolution and got 4 min 6 sec. That is just about 4X slower!
My CPU times was going to take 10 min for 8x8, 20 min 16x16, and 50 min for 32x32