Using requestAnimationFrame to approximate the browser's first paint time.

captureStartRender.js

!function(namespace) {
    // normalize requestAnimationFrame across user agents
    var vendors = ['ms', 'moz', 'webkit', 'o'];
    for(var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {
         window.requestAnimationFrame = window[vendors[x]+'RequestAnimationFrame'];
    }
    // capture first paint
    if(window.requestAnimationFrame)
        requestAnimationFrame(function(){
            namespace.firstAnimationFrameFired = new Date().getTime();
        });
}(window);

// later
if (window.firstAnimationFrameFired && window.performance)
    startRender = window.firstAnimationFrameFired - window.performance.timing.responseEnd;