make the browser struggle

struggle.user.js

// ==UserScript==
// @name        struggle.js
// @namespace   lexoyo
// @version     1
// @grant       none
// ==/UserScript==

function sleep(milliseconds) {
  var start = new Date().getTime();
  for (var i = 0; i < 1e7; i++) {
    if ((new Date().getTime() - start) > milliseconds) {
      break;
    }
  }
}
var start = Date.now();
var duration = 30000;
var end = start + duration;
function rame() {
  date = Date.now();
  sleep(1000);
  date = Date.now();
  if(date < end) {
    timerUI.innerHTML = Math.round((end - date)/1000);
    window.requestAnimationFrame(rame);
  }
  else {
    console.warn('RAME STOP');
    document.body.removeChild(timerUI);
  }

}
var date = Date.now();
console.info('RAME START', Math.round((Date.now() - date)/10)/100, 's');
window.requestAnimationFrame(rame);

var timerUI = document.createElement('p');
document.body.appendChild(timerUI);
timerUI.style.position = "fixed";
timerUI.style.zIndex = "9999";
timerUI.style.backgroundColor = "red";
timerUI.style.padding = "10px";