When I call achievement and leaderboard from GREE platform game stuck for few seconds. How to resolve this issue please help me.

We used sandbox application and my code is given below.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.SocialPlatforms;
 using System.Threading;
 
 
 public class MyClass : MonoBehaviour {
     private Gree.User localUser;
     private Gree.Achievement _achi;
         string id ;
     string userName;
     private string _result;
     // Use this for initialization
     void Start () {
     }
     
      void Update() {
                 
     }
     
     // Authentication for GREE platform
     public void Login()
     { 
         Gree.Authorizer.Login ((Gree.User user) => {
                 Debug.Log ( "Login OK : User ID = " + user.id);
             PlayerPrefs.SetString("UserID",user.id);
             PlayerPrefs.SetString("Authenticated","True");
             } , (string message) => {
                 Debug.Log ( "Login error: " + message);
             } );
     }
     
     //to save score to achivement
     public void SaveAchivement(int achiv_index)
     {
         if (Gree.Authorizer.IsAuthorized()) {
             Gree.Achievement.LoadAchievements(1, 0xFFFF, (IAchievement[] achievements) => {
             this._achi = (Gree.Achievement)achievements[achiv_index];
             this._achi.percentCompleted = 100.0;
             this._achi.ReportProgress((bool success) => {
             } );
                 
             } );
         }    
     }
         
 }

when I call SaveAchivement() from another script below, Then the game got stucked for few seconds.Is it possible to call this on background.

 var plScript: AndroidPluginConnecter;
 function Start () {
     plScript = new AndroidPluginConnecter();
 
 }
 function OnCollisionEnter (collision : Collision) { 
     plScript.SaveAchivement(14);
 }