Rigidbody load problem

I have a script:

var boxprefab : Rigidbody;  
function Update () {  
    if (Input.GetKey("up"))  
    {  
        Instantiate(boxprefab, transform.position, transform.rotation);  
        }  
    if (Input.GetKey("space"))  
    {  
        Instantiate(boxprefab, (transform.position), transform.rotation);  

    }  
}  

but when i press space or up it creates the Rigidbody for a bit and the game starts to lag and not receive any key presses until the Rigidbodys have deleted themselves. How can i make it so that it doesn't lag?

Use GetKeyDown so you only instantiate once, not every frame.