$(function() {
    $('div.menu').hide();
    $("img#menu").click(function () {
          $("div#menu").toggle("slide", {direction: "right"}, 500);
    });
});

$(function() {
    // here you are searching by class name, that is going to eat CPU
    $('div.menu').hide();
    $("img#menu").click(function () {
        // here you are doing a JavaScirpt animation, eating more of the CPU
        $("div#menu").toggle("slide", {direction: "right"}, 500);
    });
});