Simple JS timing/performance/latency testing script (via http://kellegous.com/j/2013/01/26/layout-performance/)

script.js

var Now;
if (typeof performance !== 'undefined' && performance.now) {
  Now = function() {
    return performance.now();
  };
} else if (Date.now) {
  Now = function() {
    return Date.now();
  };
} else {
  Now = function() {
    return new Date().getTime();
  }
}

var Time = function(f) {
  var s = Now();
  f();
  return (Now() - s) | 0;
};

// Useage:
var elapsed = Time(function() {
  function_to_run_that_takes_time();
});
$('#output').text(elapsed + 'ms');