The official Plex app hasn't been updated to take advantage of all the hardware decoding support available on the Apple TV. I moved to Infuse, a paid third-app, which I use to stream my Plex media content.

Jellyfin official apps are sporadic though, so Infuse is popular for Jellyfin users because it is well maintained and allows you to direct stream content without transcoding as often as possible.

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I'm also the developer of Aviato Media Server (https://aviato.media), which I just released publicly last week and is available only as a beta, and I am an ex-Plex employee.