Slow down background image scroll speed

background-scroll.js

jQuery( document ).ready( function( $ ) {
	var body = document.body,
        doc = document.documentElement;
    $(window).scroll(function () {
        body.style.backgroundPosition = "0px " + ( 830 -(Math.max(doc.scrollTop, body.scrollTop) / 4) ) + "px";
    });
});