GW2 Huge lag issues

My friend and I can run the game most of the time until one of us joins the other's session.

The hoster has a smooth performance lacking issue, but whoever joined has to deal with awful lag.

PC Deluxe Edition for both,

Ask for any specifics as I would greatly appreciate any solutions to this mess. 

(his internet may suck a bit, and he has to turn on lowest graphics settings.)