If you could attach your Jellyfin server logs, that would help so we can get a better idea of the server/client interaction. What is video codec is your media encoded with?
Jellyfin 10.10.1 (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)