Performance issue in multiplayer lobby

Hey,

me and my friend just bought the game. When we create a custom lobby, my friend slows the game down considerably. I get 200fps but he only gets like 20 and the game stutters for me aswell, while my game still shown 200fps+. We both get 250+ FPS in singleplayer and I can play fine in other multiplayer lobbies with other people.

Both our Hardwarespecs are well above the recommended ones.

Has anyone else run into similar problems and have you found a solution?