Unity3d horizontal mobile platform controller. no lag with physics. don't use it.. see https://gist.github.com/nicloay/9662874

HorizontalMovePlatformController.cs

public class HorizontalMovePlatformController : MonoBehaviour {
	public float routeLenght = 10;
	public float horizontalSpeed = 10;
	[Range (0,1)]
	public float startRelativePosition=0;
	
	public bool goRight;
	Vector3 basePosition, remotePosition;
	void Start(){
		basePosition = transform.position;
		remotePosition = getRemotePosition();
		transform.position = Vector3.Lerp(basePosition, remotePosition, startRelativePosition);
		updateVelocity();
	}
	
	Vector2 velocity;
	void Update(){
		if (goRight && (transform.position.x - remotePosition.x) > 0){
			goRight = false;
			updateVelocity();
		} else if (!goRight && (transform.position.x - basePosition.x) <0){
			goRight = true;
			updateVelocity();
		}
		rigidbody2D.velocity = velocity;
	}
	
	void updateVelocity(){
		velocity = new Vector2( goRight ?  horizontalSpeed : -horizontalSpeed, 0);
	}
	
	Vector3 getRemotePosition(){
		return basePosition + new Vector3(routeLenght, 0, 0);
	}


	Vector3 remoteVelocity;
	void OnCollisionStay2D(Collision2D collision){
		remoteVelocity= collision.rigidbody.velocity;
		remoteVelocity.x = rigidbody2D.velocity.x;
	}
	
	void OnDrawGizmos(){
		if (Application.isPlaying){
			Gizmos.DrawLine(basePosition, remotePosition);
		} else {			
			Vector2 boxSize = (collider2D as BoxCollider2D).size; 
			basePosition = transform.position;
			remotePosition = getRemotePosition();
			Gizmos.DrawLine(transform.position, remotePosition);
			Gizmos.DrawCube(Vector3.Lerp(basePosition, remotePosition, startRelativePosition),boxSize);
		}
		
	}
}