Yeah I think its the connection. With no mods, I pressed tab and when it was laggy, the bar was red and spiking to the top. We are on the same internet because we are right next to eachother. In other games my internet is fine so I dont know what to do :P
If you are on the same LAN, you can disable Steam connectivity on the coop menu and connect by IP directly. That would solve any possible issues arising from Steam side of things.
I have never done LAN connection. I tried joining through the ipv4 adress but it didnt work. I dont know if I need to change the UDP either. Am I supposed to be joining through the ipv4 adress?