Lately, and unfortunately I don’t have specifics - just some time around when I updated to tvOS 14, I have been unable to stream UHD content and sometimes run into buffering issues with high bit-rate 1080p Blu-rays as well.
I’m using a 64GB Apple TV 4K with a wired connection, and files hosted locally on my network.
When I run the speed test on a file I was having trouble streaming, it averages 170 Mbps and monitoring on the PC reports an average disk access of ~21 MB/s which lines up.
When I try to play the file I run into buffering issues and the PC reports usage closer to 8 MB/s on average - dropping as low as half that, at times.
Even if I wait for 1/4 of the movie to load in advance, it can still run into buffering issues during playback.
Switching to the legacy caching makes no difference.
It should be noted that there’s almost nothing stored on this Apple TV - I bought the 64 GB version intending to sync files locally in Infuse, before finding out that this was only an feature in the iOS version.
The only other thing on there is the YouTube app really.