Game Lagging after showing unity ads

Hi im trying to implement Unity ads in my game, In my AdManager , It has a method to show unity ads. When I testing the game in Android (Galaxy Nexus) , the game start lagging after showing the Ad. before showing the Ad there were no lags.

I'm using Unity 5.2.1

     public void ShowUnityAd (string zone)
     {
         
         ShowOptions _unityAdsShowOptions = new ShowOptions ();
         _unityAdsShowOptions.resultCallback = AdCallbackhandler;
         
         if (Advertisement.IsReady(zone)){
             Advertisement.Show (zone, _unityAdsShowOptions);
 
         }
 
 
     }
     
     void AdCallbackhandler (ShowResult result)
     {
         switch (result) {
         case ShowResult.Finished:
             Debug.Log ("AdManager ||  Ad Finished. Rewarding player...");
             GameController.controller.ContinueGame ();
             break;
         case ShowResult.Skipped:
             Debug.Log ("AdManager ||  Ad skipped. Son, I am dissapointed in you");
             break;
         case ShowResult.Failed:
             Debug.Log ("AdManager ||  I swear this has never happened to me before");
             break;
         }
     }