Can you verify if Jellyfin is remuxing without transcoding? I.e. changing container but without touching the frame/audio data.

I believe while you playback it should say in the administration panel, in the card that represents the active session you have this issue in.

Remux and transcode happen on disk, unless you manually set the temporary path to a decently sized tmpfs partition.

I solved a similar issue doing exactly what I just wrote: tmpfs (can’t recall what its name is under Docker) and set the transcoding path accordingly. I also had to tweak the transcode files’ lifetime:

• Limit trancode checked
• Remove segments checked
• Limit after: 30
• Segment lifetime: 1800

This has done wonders for me for both on-the-fly remux and transcodes, but I had to reserve a beefy tmpfs (I think I have like 8GB set right now).