iMac Pro (2017) - 64G Ram
3.2 GHz Intel Xeon W - 8 Core
Radeon Pro Vega 64 16 GB
High Sierra 10.13.2
I’m new to Blender, so this data is more anecdotal than scientific.
BMW27.blend
1920 x 1080 50%
CPU Rendering: 1:37.27
GPU/OpenCL: 3:08.51
With the CPU rendering, I could detect 12 or more patches rendering simultaneously.
With OpenCL It appears to process a single patch at a time.