WebGL game slows down after triggering javascript event

I am using Unity WebGL to host my game on a flask server and trying to throw a warning to the user whenever he tries to navigate away from the tab or mistakenly click browser's back button. Each time after the event is triggered (whenever I try to navigate away) the game slows down with a huge drop in frame rate (navigation has to be performed after the game has been loaded properly by showing the unity logo and other stuffs). Here is the script that I am using to trigger the event:

 <script>           
     window.addEventListener("beforeunload", function (e) 
         {   
             var confirmationMessage = "\o/";
 
             e.returnValue = confirmationMessage;
             return confirmationMessage;             
         });
  </script>

What is the work-around of this problem and why the game slows down completely? (I checked with firefox and chrome both !)