What is lerpSpeed?

I've used this script to rotate a cube by touch:

 private float rotationSpeed = 10.0F;
 private float lerpSpeed = 1.0F;

 private Vector3 theSpeed;
 private Vector3 avgSpeed;
 private bool isDragging = false;
 private Vector3 targetSpeedX;

 void OnMouseDown() {

     isDragging = true;
 }

 void Update() {

     if (Input.GetMouseButton(0) && isDragging) {
         theSpeed = new Vector3(-Input.GetAxis("Mouse X"), Input.GetAxis("Mouse Y"), 0.0F);
         avgSpeed = Vector3.Lerp(avgSpeed, theSpeed, Time.deltaTime * 5);
     } else {
         if (isDragging) {
             theSpeed = avgSpeed;
             isDragging = false;
         }
         float i = Time.deltaTime * lerpSpeed;
         theSpeed = Vector3.Lerp(theSpeed, Vector3.zero, i);
     }

     transform.Rotate(Camera.main.transform.up * theSpeed.x * rotationSpeed, Space.World);
     transform.Rotate(Camera.main.transform.right * theSpeed.y * rotationSpeed, Space.World);
 }

}

When i drag with the mouse when debug its ok, perfecto i think, but when i drag using touch in the phone theres a problem, when i drag its is fast and when i touch in another place of the screen without drag it rotates anyway, help please.