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?
Not sure about Jellyfin but in Emby there are user settings to disable transcoding. They’re of the same lineage so maybe a good place to look.
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.
If your use 4K usually they are encoded as x265 so it will cause transcode when in browser.
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.
would VLC better a better choice here?
Yes but then I wouldn’t be interacting with Jellyfin and wouldn’t have the Netflix-like interface, right?
from the jellyfin website discussion: When it comes to 4K SDR 10bits movies, most of the clients supports only 1080p, JellyFin transcodes it to a 1080p / 8bits file and the main quality loss is felt by that “bitmiss”.
maybe try VLC or Kodi?
