user-timing.js

/*
Strategically place:

markUserTime('some event');

Through your code to get measurements for when various activities complete.  It 
will also generate timeline events so you can see them in Chrome's dev tools.  

A good use case is to add them inline to sections of your site that are 
above-the fold (right after the menu, right after the main story, etc) and also 
in the onload handler for any critical above-the-fold images.

(coming soon) WebPagetest will report on any user timing events that were recorded 
and also expose a high-level metric for when the last user timing event fired.

With any luck we can also get RUM solutions to start reporting user timing events 
and we can significantly loosen our dependency on the (very broken) document 
onload times.
*/
;(function () {
  var object = typeof window != 'undefined' ? window : exports,
      marks = [];
  object.performance || (object.performance = {});
  object.performance.now || (
    object.performance.now = performance.now || performance.webkitNow ||
                             performance.msNow || performance.mozNow);
  if (!object.performance.now){
    var start = Date.now ? Date.now() : +(new Date());
    if (performance.timing && performance.timing)
      start = performance.timing.navigationStart
    object.performance.now = (function(){
      var nowOffset = Date.now ? Date.now() : +(new Date());
      return nowOffset - start;
    });
  }
  object.performance.mark || (
  object.performance.mark =
    object.performance.webkitMark ? object.performance.webkitMark :
    (function (label) {
      marks.push({'name':label,'entryType':'mark','startTime':object.performance.now(),'duration':0});
    }));
  object.performance.getEntriesByType || (
  object.performance.getEntriesByType =
    object.performance.webkitGetEntriesByType ? object.performance.webkitGetEntriesByType :
    (function (type) {
      return type == 'mark' ? marks : undefined;
    }));
}());

markUserTime = function(label) {
  window.performance.mark(label);
  if (console && console.timeStamp)
    console.timeStamp(label);
};