Set up iperf on a machine connected to your router.
From your machine, connect it to the router too and see if iperf shows good speed.
Take your machine to another room and plug into your lan and test again.
If the speed is considerably lower then yes you have an internal network issue. I would look into how the cables connect to the router. It might be an old or cheap switch. It could also be the cables or cable ends themselves. I’ve seen cables that are terminated poorly cause slowness. I’ve seen two connections in a single cable end work but have lots of problems.