Jesus, the difference in speed is just... wow, ridiculous. On the OpenCL part, you mean the OpenCL version when it was still supported it was even slower than today’s Cycles CPU-mode?
Yes, that's correct. I found a post on the Blender Artists forums from another person who had a similar experience as me (post is quoted below):
"I just did the same test on an iMac Pro at my local computer store. My computer at home is the first edition iMac 5K (so four years old), which I got before I planned to get into 3D (but I did get the best available GPU for it). At home the BMW render took 5 minutes on the CPU, 50 minutes on the GPU. On the iMac Pro it was 4 minutes on the CPU, 15 minutes on the GPU. Something doesn’t seem quite right. On the other hand, there is the AMD ProRender plug-in for Blender, which should work better, although I didn’t get the chance to try it out.
I am also told that you can download the latest OpenCL drivers directly from AMD, which surprised me, because I didn’t think you could manually install drivers on the Mac OS (I assumed you had to wait for Apple to release the next version of the OS)."