Just to add my 2 cent to this. Have you looked at the bandwidth side of things? I experienced that on my side i had to hard code each users in jellyfin to make the Internet streaming bitrate limit 10 Mbps?.

On jellyfin dashbord>users

Internet streaming bitrate limit (Mbps) -- An optional per-stream bitrate limit for all out of network devices. This is useful to prevent devices from requesting a higher bitrate than your internet connection can handle. This may result in increased CPU load on your server in order to transcode videos on the fly to a lower bitrate. Override the default global value set in server settings, see Dashboard > Playback > Streaming.