headSelectorTest.user.js

// ==UserScript==
// @name			Header Selector Test
// @author			Erik Vold
// @namespace		headSelectorTest
// @include			http*://*
// ==/UserScript==

(function(){
	function test(){
		var head,diff=[],start,iterations=100000;
		if(document.documentElement.firstChild.tagName!="HEAD") return alert("document.documentElement.firstChild failed " + document.documentElement.firstChild.tagName);

		start = (new Date).getTime();
		for(var i=0;i<iterations;i++){
			head=document.getElementsByTagName("head")[0];
		}
		diff[0]=(new Date).getTime() - start;

		start = (new Date).getTime();
		for(var i=0;i<iterations;i++){
			head=document.documentElement.firstChild;
		}
		diff[1]=(new Date).getTime() - start;

		alert(diff[0] +" - "+ diff[1] + ", avg diff = " + (diff[0]-diff[1])/iterations);
	}
	GM_registerMenuCommand("Header Selector Test",test);
})();