I get big performance hit when there is more then 3 player in a server.
The same thing is happening wether I am host or not.
I don't have awesome computer specs, but if I run the same mission in single player or in mutiplayer with no one joining, then It runs smoothly.

My friend is having the same issue although he have a way better computer then me. So it seems unrelated to actual spec or the computers.

This is weird because if I play with a group of 3 AI folowwing me, then there is no performance hit.