Thank you very much, this is very clear.
I have read a few articles about how to handle latency exposing some mehtods to deal with that : prediction, reconciliation, interpolation.
http://www.gabrielgambetta.com/client-side-prediction-live-demo.html
For now, I have tested your extension with two computers on the same internet connection. Each one has a different steam account. I dont have implemented any of the methods cited before (prediction ,reconciliatiob, interpolation) but I do not see any lag for now. I wonder if the packets are send locally or if they transit trought steam. In the first case, this is not good because I am not testing the game in real conditions. Then how is it possible to test a multiplayer game with gamemaker with simulated lag ?