Best way to reduce lag on LAN?

I would disconnect or turn OFF all other devices as much as you can accept for the xbox and game. See if you notice any lag. Directly connect your game console to the router. Don't use a switch.

If your experiencing LAG while other devices are online and you don't when everything else is off, then one of these devices is probably the culprit. You'll need to graduate turning on each device to see when and where the LAG starts. One thing to look at if you have a switch that has LED indicators is to see if the switch LED actifivity is constantly ON and blicking when you feel that nothing else is going on, or not gaming or streaming anything. If this is happening, then one of the devices is doing something it shouldn't be.

Seems like this is not a router issue.