unity LoadSceneAsync works like LoadScene

So I have t$$anonymous$$s code :

 AsyncOperation async;

 public void Start ()
 {
     StartCoroutine (LoadingScreen ());
 }

 IEnumerator LoadingScreen ()
 {
     yield return new WaitForSeconds (1); 

     async = SceneManager.LoadSceneAsync (1);
     async.allowSceneActivation = false;

     w$$anonymous$$le (!async.isDone) {
         if (async.progress == 0.9f) {
                 async.allowSceneActivation = true;
         }

         yield return null;
     }
 }

The game still freezes before it loads another scene. I want the game to still smoothly load w$$anonymous$$le it is loading a different scene, how can I do that? Have I done anyt$$anonymous$$ng wrong with my code?