Based on some digging I was doing a few weeks ago, the HEVC hardware acceleration was not enabled via eGPU. That could obviously change "this Spring" with whatever further enhancements Apple is doing to better support eGPUs.
Just did some test with that video. I think there is nothing to do with the GPU acceleration, but just because the iMac Pro has a stronger CPU.
When I use Preview and QuickTime Player. The CPU usage is low anyway. And this is on my ancient Mac Pro 5,1 with the old Xeon. The video is definitely not smooth, but it plays.
Preview: The CPU load history shows that the CPU is intermittently used, every time when the usage drop to zero, the video will be freeze for a moment (choppy).
QuickTime Player: Basically the same as Preview (of course, preview is actually using the QuickTime player in the background)
IINA: playable, not smooth, and high CPU usage.
VLC: Low CPU usage, but there is no "video" at all, it's always stay at the same frame. Click on the time line will move to a new frame, music normal, but no video.
Even on my W3690, the average CPU usage to play this video (in QuickTime) is just above 20%. I can imagine on the iMac Pro, the CPU usage can further lower to about 10%. Which actually match your observation.
Also, it seems this video is actually not that demanding (In QuickTime), but it's the 10bit causing trouble in the old system. I downloaded the little bit higher bitrate but 8bit video sample. My Mac can actually play it smoothly with more or less the same CPU usage (except VLC, still no "video" at all).
Up to this stage, I tend to believe that there is no hardware video acceleration (H264 and H265) on the iMac Pro (in MacOS). But simply because the CPU is "too strong", and QuickTime is coded to "use less resource", but not "smooth playback". Therefore, create an illusion that looks like iMac Pro has H265 hardware decode.
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nightly builds of VLC play HEVC videos just fine ->
https://nightlies.videolan.org/build/macosx-intel/
Thanks, just tried the latest V4 nighty build. Still no joy with the 10 bit sample.
But 8bit can play smoothly with around 600% CPU, better then IINA actually.