Movement (left/right) and jumping = laggy motions.. please help!

Alright. So i have this game - like subwaysurfer ! with my ball stuck in z position and only being able to move it in fixed positions left and right and jumping. Having the level with objects moving towards the player. you know what i mean.

I have everything working with two scripts.. but both of them combined doesn't seem to work. the motion when jumping is laggy and it seems like there is a problem with framerate. idk. please help .

this is my script for movement: function Start () { }

  var speed : float = 3.0;
  var positions : Vector3[];
  
  private var isFalling = false;
  private var index = 1;
   
  function Update()  {
  
      if (Input.GetKeyDown(KeyCode.LeftArrow) || Input.GetKeyDown(KeyCode.A))  {
          index--;  
          if (index < 0) index = 0;
      }
   
      
      if (Input.GetKeyDown(KeyCode.RightArrow) || Input.GetKeyDown("d")) {
          index++;
          if (index >= positions.Length) index = positions.Length-1;
      }
       {
       transform.position = Vector3.MoveTowards(transform.position, positions[index], Time.deltaTime * speed);
     }
 }
 
 function OnCollisionStay ()
     {
     isFalling = false; 
     }
  

and this is my script for jumping:

 var JumpH = 8;
 var isFalling = false;
 
 function Update () {
     if (Input.GetKey(KeyCode.Space) && isFalling == false)
     {
         rigidbody.velocity.y = JumpH;
         isFalling = true;
     }
 
 }
 
 function OnCollisionStay () {
 
     isFalling = false;
 
 }