I have an iMac Core i5 3.7 GHz with a Vega 48 and I tried lately VideoProc on it. The company developing it says that it supports almost all the newer GPUs for hardware acceleration. However, in this particular iMac model it detects only the Intel HD graphics, completely ignoring the Vega 48. Could you (iMac and iMac Pro users) please install this multi-purpose video converter and check what does it report in the hardware acceleration settings? Does it see and report correctly your GPU? The trial version has some limitations but it will let you check the settings and even do conversions of short videos. Thanks!