Blender 2.79
Settings:
1080p @ 50%
CPU: Ryzen 5 1400 3.58 Ghz
GPU: Radeon RX 580
Run 1:
Center: 128 x 128
Time: 1:30 (GPU)
Time: 3:50 (CPU)
Run 2:
Center: 512 x 512
Time: 1:27 (GPU)
Time: 12:25 (CPU)
Run 3:
Center: 256 x 256
Time: 1:28 (GPU)
Time: 4:36 (CPU)
Run 4:
Center: 64 x 64
Time: 1:46 (GPU)
Time: 3:37 (CPU)