Prefer local connection on LAN

[I-G-1-1]

On your router (or on the device that you use as DNS server on your LAN) add a line to the hosts file: ip-of-the-machine-running-jellyfin external-url

At this point even if you use the URL any client inside the LAN that uses the router as DNS server will connect directly to the ip-of-the-machine-running-jellyfin without exiting to the internet.

If any of the client devices use some sort of DNS "protection" that ignores the router DNS you have to change the hosts files in each client devices