To help narrow down whether it's something to do with that new computer causing a local network problem, does your PowerShell script still show problems with packet loss/high latency with the new computer physically disconnected?
That'll help us narrow down the potential causes - if the latency problem persists with the computer physically disconnected, we'll start looking at the router/switch, and if it goes away, we can focus on the PC and the port that was being used.