I'm out of ideas then. I was thinking maybe there was some sort of situation where your HDD wasn't able to keep up with the IO, causing the CPU usage to spike (this happens a lot of users running JF on a RPi).
Jellyfin 10.10.3 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)