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/
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You could disable transcoding. But depending on what device you are streaming to this may not be possible. The device would have to support decoding of whatever the stream is encoded as. After several issues with Quicksync errors with an older Intel CPU, I finally tossed an old 1050ti in my Emby box. I patched the driver so it can handle more 1080p streams (https://github.com/keylase/nvidia-patch). It is solid, fixed all of my buffering/stuttering/artifacting issues and I think it can do 2-3 simultaneous 4k transcodes.