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?
Are you transcoding? If so, need to make sure your CPU or GPU are up to he task.
How can I actually avoid the transcoding?
You can disable and tweak transcoding settings, but it might mean that certain media isnt going to be able to be played at all, if its a certain format. If you have a beefy computer, this shouldnt be an issue, but if not id check this out: https://jellyfin.org/docs/general/server/transcoding/