[noggin]

OK - an update - found the Codec info screen from settings in MrMc.

I have a feeling the Apple TV 4K may be hardware decoding this 4:2:2 2160p50 HEVC? The sample clip plays in MrMC (with terrible A/V sync) with a reported CPU load of just 11%, and the video codec is reported as 'hevc (Rext)'. There were 14 skipped frames, none dropped.  (Could 'Rext' suggest it's being played by an external hardware player in tvOS?)

My AVCi100 100Mbs 4:2:2 1080i25 (containing a 1080p25 source) plays OK, albeit with a reported CPU ~201% load, and is reported as 'h264 (High 4:2:2 Intra)'. There were 3 skipped frames, none dropped. (This doesn't play reliably - sometimes it refuses to play)

(Yes I realise MrMC isn't Kodi - but the Apple TV 4K is a very versatile and powerful platform - and one of the only platforms that has automatic frame rate switching on the major DRM services like Netflix and Amazon Prime...) It also has limitations with regard to HD Audio bit streaming (no DTS:x or Dolby True HD with Atmos), and also has 23.976 vs 24.000 and 59.94 vs 60.00 issues in some situations AIUI?

Sorry forgot to update about this.  Yes - MrMC does play 4:2:2 2160p50 HEVC 10-bit content in hardware.  Apparently Apple have implemented HEVC decode with 4:4:4 and 4:2:2 support as well as the normal 4:2:0 used for consumer video.  One factor is so that they can support Sidecar (their iPad-as-a-second screen) system using HEVC compression but avoiding reducing the chroma resolution on desktop displays (otherwise 4:2:0 would give you 1/4 chroma resolution - which would make colour on your Mac desktop viewed on your iPad look smeary and low quality - so they use 4:4:4 ?)