I noticed after the last update that live tv wasn’t as smooth as usual - I went to the deinterlacing settings and saw that linear now says 60fps. TV in the UK is 50fps and I have my Apple TV set to 50fps. Any chance of a 50fps linear setting?
The linear setting doubles the native framerate, so it’s already 50fps for you.
Likely you are hitting the known performance issue in tvOS 11.0, which is fixed in the tvOS 11.1 Beta.