Random sampling of page load time using Boomerang - http://lognormal.github.io/boomerang/doc

boomerangsampling.js

<script defer type="text/javascript">

//set random variable
var x = Math.floor((Math.random()*100)+1);


//check to see whether user has tracking cookie set
if (document.cookie.search(/(^|;)perfTrack=/) > -1) { 

  //if tracking cookie has been set, create alert with page load time
  BOOMR.subscribe('before_beacon', function(o) {
      
    alert("hello again, your page loaded in " + o.t_done + "ms");
    
  });
  
}


// if no tracking cookie exists, check to see whether random variable falls within set range (this example checks 10%)
else if (x >= 30 && x <= 39) { 

  // set a new cookie to track further page views
  document.cookie = "perfTrack=true;max-age=" + 60 ; //  60 seconds to a minute, * 60 minutes to an hour, * 24 hours to a day, * 365 days.
  
  // create alert with page load time
  BOOMR.subscribe('before_beacon', function(o) {

    alert("tracking initiated, your page loaded in " + o.t_done + "ms");

  });

} 


// if no tracking cookie exists and variable is outside sample range, do nothing
else { 

  alert("no tracking " + x);

}

</script>