Why Jellyfin doesn't just 'work'?

[TheDreadPirate]

Do you have hardware acceleration turned on? It is software transcoding because the subtitles are not supported by the Android TV app (this is common). But your CPU is doing the transcoding and not doing it fast enough.

frame= 55 fps=1.2 q=28.0 size=N/A time=00:00:02.43 bitrate=N/A speed=0.0511x

Settings > Dashboard > Playback

You can use either Intel Quick Sync or NVENC since you have both. I would use Quick Sync.