var cpuMeasurement = function (timeFrame) { // timeFrame in milliseconds
  var cpum = 0;

  var start = new Date().getTime();
  var end   = new Date().getTime();

  while (end - start < timeFrame) {
    cpum++;
    end = new Date().getTime();
  }

  return cpum;
};

console.log(cpuMeasurement(20));