why does attaching a script to unity makes the game go laggy?

I'm not sure if the script is the problem since it works perfectly when you restart. But here is it anyway.

     Rigidbody2D rb;
     Transform target;
     public float rotateSpeed = 200f;
     
     IEnumerator WinBlock()
     {
         GetComponent<Animator>().SetBool("explode", true);
         yield return new WaitForSeconds(0.5f);
         Destroy(this.gameObject);
     }
 
     void Start()
     {
         if (!(GameObject.FindGameObjectsWithTag("enemy").Length == 0))
         {
             target = GameObject.FindGameObjectWithTag("enemy").transform;
 
         }
 
         rb = this.gameObject.GetComponent<Rigidbody2D>();
         transform.Rotate(0, 0, -90); // this makes the missile spawn horizontally
     }
     private void FixedUpdate()
     {       
         Vector2 direction = (Vector2)target.transform.position - rb.position;
         direction.Normalize();
         float rotationDir = Vector3.Cross(direction, transform.up).z;
         rb.angularVelocity = -rotateSpeed * rotationDir;
         rb.velocity = rb.transform.up * 8;
     }
     private void Update()
     {
         //if there's no enemy tag the missile explodes
 
         if (GameObject.FindGameObjectsWithTag("enemy").Length == 0)
         { 
             StartCoroutine(WinBlock());
         }
     }