Simple multiplayer game lag

It's because every client is controlling everyone else. On your controller script, you need to add:

 if(networkView.isMine){
 
    //Movement controls
 
 }

You will also need to do this for every other inputs you have for your character. Only then will the user be moving his character, and only his character.

I had a problem like this a few days ago.

For every player that joined, It'd spawn a sphere that the player could control in 3rd person. But it appeared as though it had lag. By an accident, one of the spheres stopped moving as I kept moving the other player's sphere. I noticed that the control inputs from one player also attempted to move the other spheres.

Say I would push W, my sphere would move forward, and the other spheres would start to jitter as though they attempted to go forward, but were not allowed.

I think I solved this problem by deactivating every script attached to the sphere, and then activated them when for the instantiated object when the player spawned.

Something like this.

 function SpawnPlayer(player : NetworkPlayer){
     var _NewPlayer : GameObject = Network.Instantiate(PlayerPrefab, transform.position, transform.rotation, _PlayerNumber);
     _NewPlayer.GetComponent(moveCharacter).enabled = true;
     _NewPlayer.GetComponent(shootCubes).enabled = true;
     _NewPlayer.GetComponentInChildren.<Camera>().enabled = true;
 }

This was a tricky problem that took me a while to detect and solve. I only just got into networking and I still don't fully understand the concept, but hopefully this help =)

Try this:

Don't sync/ monitor anything with the NetworkView.

Don't use rigid body physics for the player characters in a 2d platformer- never mind that it's a networked game. Just don't. Animate them using fake physics. You can use physics in other places in the game, just not for the main characters.

Have each player RPC their position and velocity as two Vector2s; and have a single GameController object with the only NetworkView on it and a script to receive all these RPCs and position all the remote player's characters accordingly.

Don't use Network Instantiate, send the server an RPC that you are whomever and you just made a whatever, and have it run a function to send RPCs to all the other clients telling a function on their GameController script to make a new whatever that belongs to whomever.

It won't be perfect, but I bet this will already look a lot better than what you are doing and be a good place to start.

One other thing: Think about the tree falling in the forest with no one around to hear it. If you are too far from another player to collide with them any time soon, or to shoot them before they move out of range- does it really matter if they really are where they appear to be at that exact moment? Prioritize what you must keep close track of (where there must be lag, make it count) and fake the rest client-side to make it look as lag-free as possible.