I am running jellyfin in LAN on a Synology. I/O from harddisk shouldn’t be a problem being SSD on the same NAS. And WiFi should sustain a 4K stream. I have tried only web clients. How can I optimize the stream for such large file? Or how can I debug the bottleneck?
Depending on source file format and which client you’re using, it may be transcoding due to the client being unable to natively play the file.
If you don’t have the option of using hardware acceleration like Intel Quicksync, then the best option is trying to find a better client, or transcoding the source file manually before hand.