The forEach includes many checks internally and isn't as straight forward as a simple loop.
See the Mozilla Javascript reference for the details:
if (!Array.prototype.forEach)
{
Array.prototype.forEach = function(fun /*, thisArg */)
{
"use strict";
if (this === void 0 || this === null)
throw new TypeError();
var t = Object(this);
var len = t.length >>> 0;
if (typeof fun !== "function")
throw new TypeError();
var thisArg = arguments.length >= 2 ? arguments[1] : void 0;
for (var i = 0; i < len; i++)
{
if (i in t)
fun.call(thisArg, t[i], i, t);
}
};
}