Edit; oops, I thought you talked about both Apple and Android, I’m so used to ATV being Android tv.
I might leave the comment anyway if it helps someone else.
For Android tv, if using the official app, I would recommend downloading “Just (Video) Player” and then configuring the Jellyfin app to use external player.
On my Shield the videos play much better without transcoding
Edit; oops, I thought you talked about both Apple and Android, I’m so used to ATV being Android tv. I might leave the comment anyway if it helps someone else.
For Android tv, if using the official app, I would recommend downloading “Just (Video) Player” and then configuring the Jellyfin app to use external player. On my Shield the videos play much better without transcoding