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(function(w){
	var perfNow;
	var perfNowNames = ['now', 'webkitNow', 'msNow', 'mozNow'];
	if(!!w['performance']) for(var i = 0; i < perfNowNames.length; ++i)
	{
		var n = perfNowNames[i];
		if(!!w['performance'][n])
		{
			perfNow = function(){return w['performance'][n]()};
			break;
		}
	}
	if(!perfNow)
	{
		perfNow = Date.now;
	}
	w.perfNow = perfNow;
})(window);

// Usage
console.log(window.perfNow());

Just a quick performance.now() polyfill since the ones I found were producing "Illegal invocation" errors. This polyfill wraps the performance call inside a function to avoid this.

Performance.now() polyfill on GIST