As it happens exactly when another player joins, look at the network/router side before blaming the laptop hardware. The fact that joining someone else’s game works normally also points in that direction. The upload dropping below 1 Mbps is the biggest clue. I’d check whether the router is struggling with NAT/UPnP or bufferbloat when the game starts sending more traffic. Since you already tried wired, Wi-Fi and a phone hotspot, I’d also test with all VPNs, third-party firewalls and network monitoring software disabled. If the problem still happens through the phone hotspot, that would make a normal router configuration issue less likely and I’d focus more on the game’s hosting/network code or the Windows network stack.
