It’s different for me. I have 2 Apple TV boxes. I have an Apple 4K in my living room and then my Apple TV from 2 years ago in the bedroom. The one in the bed room runs fine. The 4K studders like no ones business. Audio will play, but video studders like it is freeze framing through the video. If I throw the video up to the apple 4K tv through my iPhone from the site, it works alright. This has been going on for almost what seems like 2 months now.
I have the same issue with my Apple TV 4k and it has been going on since the latest OS update. Audio plays fine but video is a slideshow.
Are you using the Apple TV app, a browser, another app? I'm not sure I have a 4k Apple TV for testing but I can see if I can replicate.
I am using the Apple TV app on tvOS 13.2. It has been an issue since tvOS 13.