SOLVED: Hardware Acceleration

[TheDreadPirate]

You are hardware transcoding. And ffmpeg going that fast is necessary because you do want to keep ahead of where you are watching and build up a buffer. There is transcode throttling in Dashboard > Playback (near the bottom), but not in the way you are thinking. With throttling enable once ffmpeg gets far enough ahead, jellyfin will pause ffmpeg. Once the playback starts to catch up jellyfin will unpause ffmpeg and it will transcode some more at full speed.