SceneManager.LoadSceneAsync loaded scene takes too long to open

I'm trying to make sort of a progression bar of a scene I'm loading in the background and that when it's fully loaded open it. The moment the bar reaches 100% it opens the scene. The problem is that when it reaches 100% and starts opening it, it takes too long to actually open (like 5 - 7 seconds after it has fully loaded). The application is for android, I've tested it on the newest phones and it's the same. Here is the code I'm using:

 using UnityEngine.SceneManagement;
 using UnityEngine.UI;
 using System.Collections;
 
 public class MenuApp : MonoBehaviour {
 
     AsyncOperation ao;
     public GameObject loadingObject;
     public Image loadingImg;
     public Text loadingText;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
     public void LoadARScene(){
         loadingObject.SetActive (true);
         StartCoroutine (LoadARSceneInBG ());
     }
 
     IEnumerator LoadARSceneInBG(){
         yield return new WaitForSeconds (1);
 
         ao = SceneManager.LoadSceneAsync (1);
         ao.allowSceneActivation = false;
 
         while (!ao.isDone) {
             loadingImg.fillAmount = ao.progress;
             loadingText.text = (ao.progress * 100).ToString("F1") + "%";
 
             if (ao.progress == 0.9f) {
                 loadingImg.fillAmount = 1;
                 loadingText.text = "100%";
                 ao.allowSceneActivation = true;
             }
             yield return null;
         }
     }
 }
 

Maybe someone can help me with this issue??? Thanks...