CPU

[TheDreadPirate]

There are multiple ffmpeg processes, which tells me this is either a library scan or trickplays generating.

If you see this again, run this command and share the output.

Code:

sudo ps -aux | egrep "jellyfin|ffmpeg" | cat

If these are ffmpeg processes related to either a library scan or trickplays, there are settings to cap the number of threads.

Dashboard > General > Parallel library scan tasks limit/Parallel image encoding limit
Dashboard > Playback > Trickplay > FFmpeg Threads