2D platformer IEnumerator problems

I want my hero to be slowed down for 2 seconds by enemy bullet.

  public float speed = 3f;
  

And on collision with the bullet I call:

 private void OnCollisionEnter2D(Collision2D collision)
     {
         if (collision.gameObject.tag == "AnorexBullet")
         {
             speed = 1f;
             Debug.Log("Hit");
             StartCoroutine(SpeedDisabler());
             speed = 3f;
         }
 
     }
     IEnumerator SpeedDisabler()
     {
         yield return new WaitForSeconds(3);
     }

The problem I have that the speed won't change... What am I missing?