...shows many computers/devices (with neighbor's name) as ethernet (not wireless)?
performance has been slow.
What kind of internet service and router do you have?
How far are these neighbors, People have done some pretty creative things to steal internet from their neighbors. If you are hooked up ethernet WAN (look at your router lights and see which WAN is lit up) and your ONT is outside your house, they could have done something like that. If you are COAX WAN then that becomes less likely.
In the router interface you can hit block internet, and they won't get internet. that's a quick fix, but you may want to inspect the outside ONT if you have one and see if anything is being re routed or split with a hub or switch.
If you google 'neighbors stealing internet' you'll see there are millions of hits, and you have to dig down for wired ethernet theivery.
Are you in a MDU (apartment, coop etc)?
Other possibility that comes to mind is is that you have mistakenly logged into their wireless router.