How to optimize game performance in this code?

HI, i'm creating a game, in it my ball keep moving inside a box(4 walls), but it is not moving smoothly. I mean may be FPS are low or somehhing else out of my mind. What i want is, below code to be reset to optimize my game performance from the ball perspective. What changes should i make?

public class PlayerScript : MonoBehaviour { public UIscript uiScript;

 public GameObject PlayerWall;
 public GameObject Player;
 public GameObject Buondary;

 public bool tapped = false;
 
 public    float speed = 0.9f;
 public Vector3 velocity;
 
 Vector3 scale;

 void Start () 
     {
     scale = uiScript.txt_currentscore.transform.localScale;

         uiScript.GameOn = true;
         uiScript.TapToPlay.SetActive (true);
         uiScript.ReplayPanel.SetActive (false);
         velocity = new Vector3 (Random.Range(-5,5), Random.Range(5,8), 0.0f);
         PlayerWall.SetActive (false);
     }

 void Update ()
     {
         if (Input.GetMouseButtonDown (0))
             {
                 tapped = true;
                 uiScript.TapToPlay.SetActive (false);

                 if(uiScript.GameOn)
                 {
                         StartCoroutine (EnableWall ());
                 }
             }
     }

 IEnumerator EnableWall()
             {
         uiScript.tapAudio.Play ();
         PlayerWall.SetActive (true);
         yield return new WaitForSeconds (0.15f);
         PlayerWall.SetActive (false);
         StopCoroutine (EnableWall ());
             }

 void FixedUpdate()
             {
                 if (tapped) 
                     {
                         if(velocity.x != 0)
                         {
                         uiScript.selected.rigidbody.MovePosition
                             (Player.transform.position+velocity * Time.deltaTime *speed);
                         }
                 else
                 {
                     velocity += new Vector3(1,1,0);
                                     uiScript.selected.rigidbody.MovePosition
                                             (Player.transform.position+velocity * speed *Time.deltaTime);            
                 }
                 uiScript.selected.gameObject.transform.Rotate(new Vector3(0,0,velocity.y),Time.deltaTime*speed*200);
             
                     }
             }

 void OnCollisionEnter(Collision collision)
             {
                 uiScript.collideAudio.Play ();            

                 foreach (ContactPoint c in collision.contacts) 
                         {
                             velocity = Vector3.Reflect (velocity, c.normal);
                             velocity.z = 0;
                         }

                 if (collision.collider.tag == "wall") 
                     {
                         uiScript.score += 1;
                         uiScript.UpdateScore();
                         if(speed < 3.8f)
                             {
                             speed += 0.2f;
                             }
                     }
             }

 void OnTriggerEnter(Collider other)
         {
             uiScript.GameOver ();
         }

}