Ok, to summarize, tell me if I am wrong, what I have to do :
1°) Send changed state variables to server only when it is triggered (for exemple, one client press up, , then u send the new y position of the object) and not every frame because it costs too much
2°) Use extrapolation and interpolation on client side in order to smooth a little bit the movement of other players
3°) Dont use lag compensation because it is a small game.
Finally, I wonder if it wouldnt be better to use fast protocole (UDP) instead of reliable (TCP) ? Indeed, with interpolation and extrapolation , it is not a problem anymore if a few apckets are missing ? What do u think ? In ur example, u have set everything to reliable.
Anyway, thank you a lot, I will try that.