Mac performance using gpu is abysmal. Why?

I work in a mixed win and mac environment but would love to move to mac only since I approciate the macos.
My main workstation right now is a win10 machine with i7 5820 + dual vega 64 which has great performance but some instability issues.

I went down to the local mac store and tried out the 8-core vega 56 model imac pro in blender 2.79 official. Cpu performace was similar to my existing 5820 but gpu performance was abysmal. Many times slower than a single vega 64 in win10. What is the reason? Is it the old 1.2 version of opencl on macs or some other reason? I planned to move my existing vega 64s to external gpu-enclosures and use from the imac pro but now it seems that is not a good idea.

Any other that have experiences with mac and opencl performance?