I ran many different render times, and here are my best ones.
GPU = Radeon R9 390
CPU = Ryzen 1700
For my GPU
Tiles = 512x512
Time: 1 minute 30 seconds (GPU - OPENCL)
For my CPU
Tiles =16x16
Time: 1 minute 54 seconds (CPU)
The difference between 512x512 and 256x256 on GPU is 1 second
Likewise, the difference between 16x16 and 32x32 on CPU is 1 second.
So overall my GPU does a better job, my CPU does well but cannot compete with it.