SOLVED: Intermittent Extremely Slow Stream Speeds on LAN only

[St. Glocktopus]

@TheDreadPirate

Welp, I think I found the solution and it's a strong maybe but it's working.

I made some config changes to my apache/site-available/000-default-le file and disabled the recommended SSL protocols from Jellyfin's KBs and re-enabled the default SSL protocols from /etc/letsencrypt/options-apache file.

Now I can connect to the server via localIP:8096 and get full speeds of 1Gbps on LAN. I'm not going to continue perusing the issue for why the FQDN causes stuttering on the local network as I have direct access again.

Thank you for helping me narrow down the issue and eliminate the Jellyfin server itself as the root issue. I spent far too long thinking Jellyfin itself was the issue and ignoring the fine details in Apache.
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If anyone encounters a similar issue with this similar setup please feel free to DM me and I'd be happy to help try to resolve it.

[gycklarn]

Hello.

First of all I would like to apologize for resurrecting an almost two year old post, but it's one of the first posts I found when googling for my issue and this might be found by other users with the same problem.

I believe I have found the actual solution for OP's problem, not just a workaround.

I had a problem very similar to OP. My friends are able to stream in max quality with no buffering issues at all, but I was experiencing major issues when trying to stream from LAN by connecting to the domain name that pointed to the Jellyfin server. I could remedy this by instead connecting to the LAN ip and specify port, just like OP. In addition, just like OP, I also have a router from TP Link. The Deco M9, to be precise.

When specifying and connecting to the domain name (as opposed to the LAN IP address), I saw how the transfer speed dropped from 10MB/s to approximately 250kB/s after 5-10 minutes.

I went through a bunch of troubleshooting steps (including mistakenly blaming NginX which I use as a reverse proxy), but I'll skip past them to the actual solution:
In the router's QoS settings, I had a few old clients configured as "prioritized devices". Simply removing all of these immediately solved the problem. I was streaming a movie while troubleshooting, and I saw the transfer speed immediately go back to 10MB/s. I was worried it was just temporary, or that the router just "reloaded" and reset before throttling the transfer speed again, but it's been several days since my change and everything is still working fine.

I have no idea why the QoS settings would lead to this behaviour, all I know is it apparently did. I have no actual use for QoS, it was just something I did as an experiment when I got the router several years ago and forgot about.

So, OP and any other user that might have the same issue: Check your QoS settings in your router's control panel. If you have any devices specified, try to simply disable QoS in its entirety.