A simple snippet you can add to your bookmark bar (insert javascript: in front) to measure rendering performance of a state of a webpage

bookmarklet-version

javascript:(function (window,$,runs) { var t = new Date(), i = 1, timetaken,offset = $(window).scrollTop(); $(window).bind('scroll.scrolltest',function () { if (i >= runs) { timetaken = (new Date) - t; console.log('Time taken for '+runs+' scrolls: ',timetaken,' - time per scroll: ', timetaken/runs); $(window).unbind('scroll.scrolltest'); return; } i++; doScroll(); }); function mod (x,y) { return Math.round((x/y - Math.floor(x/y)) *2); } function doScroll () { setTimeout(function(){ $(window).scrollTop(offset + mod(i,2)*100); },0); } doScroll(); })(window,jQuery,500);

scrolltest.js

// Note: this requires that the jQuery object ($) is present on the page
// The stats are displayed in the console
(function (window,$,runs) {
    var t = new Date(), i = 1, timetaken,offset = $(window).scrollTop();
    $(window).bind('scroll.scrolltest',function () {
        if (i >= runs) {
            timetaken = (new Date) - t;
            console.log('Time taken for '+runs+' scrolls: ',timetaken,' - time per scroll: ', timetaken/runs);
            $(window).unbind('scroll.scrolltest');
            return;
        }
        i++;
        doScroll();
    });
    // was having problems linking to the bookmarklet, due to the modulus operator, so I need to hack my
    // own w/o using an actual modulus
    // also making sure the number's round, in case some browsers have retarded floating point calcs
    function mod (x,y) {
        return Math.round((x/y - Math.floor(x/y)) *2);
    }
    function doScroll () {
        // I had to do this really ugly hack to make the scrolling always get triggered - not sure why
        // posssible reasons might be that the scroll event doesn't get triggered on every scroll and/or
        // there's some sort of ratelimit done by the browser to improve scrolling performance on
        // Javascript
        setTimeout(function(){
            $(window).scrollTop(offset + mod(i,2)*100);
        },0);
    }
    doScroll();
})(window,jQuery,500);

Omg, I was left squinting watching the screen flicker xD

Still, nice snippet!