Javascript JS/Jquery-视差滚动效果滞后且缓慢

我有一个视差功能的幻灯片部分，我的网页有3个背景图像。由于某些原因，当我滚动时，它显得非常滞后和缓慢，有时很难滚动到第三个背景图像，或者如果我已经在第三个背景，我似乎无法再次滚动。如果我滚动到第一个背景，我似乎被困在这一部分，我不能去上面的部分。不知道我哪里出错了

以下是我的js：

    (function ($) {
        var types = ['DOMMouseScroll', 'mousewheel'];
        if ($.event.fixHooks) {
            for (var i = types.length; i;) {
                $.event.fixHooks[types[--i]] = $.event.mouseHooks;
            }
        }
        $.event.special.mousewheel = {
            setup: function () {
                if (this.addEventListener) {
                    for (var i = types.length; i;) {
                        this.addEventListener(types[--i], handler, false);
                    }
                } else {
                    this.onmousewheel = handler;
                }
            }
            , teardown: function () {
                if (this.removeEventListener) {
                    for (var i = types.length; i;) {
                        this.removeEventListener(types[--i], handler, false);
                    }
                } else {
                    this.onmousewheel = null;
                }
            }
        };
        $.fn.extend({
            mousewheel: function (fn) {
                return fn ? this.bind("mousewheel", fn) : this.trigger("mousewheel");
            }
            , unmousewheel: function (fn) {
                return this.unbind("mousewheel", fn);
            }
        });

        function handler(event) {
            var orgEvent = event || window.event
                , args = [].slice.call(arguments, 1)
                , delta = 0
                , returnValue = true
                , deltaX = 0
                , deltaY = 0;
            event = $.event.fix(orgEvent);
            event.type = "mousewheel";
            if (orgEvent.wheelDelta) {
                delta = orgEvent.wheelDelta / 120;
            }
            if (orgEvent.detail) {
                delta = -orgEvent.detail / 3;
            }
            deltaY = delta;
            if (orgEvent.axis !== undefined && orgEvent.axis === orgEvent.HORIZONTAL_AXIS) {
                deltaY = 0;
                deltaX = -1 * delta;
            }
            if (orgEvent.wheelDeltaY !== undefined) {
                deltaY = orgEvent.wheelDeltaY / 120;
            }
            if (orgEvent.wheelDeltaX !== undefined) {
                deltaX = -1 * orgEvent.wheelDeltaX / 120;
            }
            args.unshift(event, delta, deltaX, deltaY);
            return ($.event.dispatch || $.event.handle).apply(this, args);
        }
    })(jQuery);

    var Parallax = {
        utils: {
            doSlide: function (section) {
                var top = section.position().top;
                $('#section-wrapper').stop().animate({
                    top: -top
                }, 600, 'linear', function () {
                    section.addClass('slided').siblings('div.section').removeClass('slided');
                });
            }
        }
        , fn: {
            setHeights: function () {
                $('div.section').height($(window).height());
            }
            , onSiteScroll: function () {
                var section = null;

                $('#section-wrapper').mousewheel(function (event, delta) {
                    event.preventDefault();
                    if (delta < 0) {
                        section = ($('.slided').length) ? $('.slided') : $('#section-1');
                        var next = (section.next().length) ? section.next() : $('#section-1');
                        Parallax.utils.doSlide(next);
                    } else if (delta > 0) {
                        section = ($('.slided').length) ? $('.slided') : $('#section-1');
                        var prev = (section.prev().length) ? section.prev() : $('#section-1');
                        Parallax.utils.doSlide(prev);
                    }
                });


            }
        },

        init: function () {
            for (var prop in this.fn) {
                if (typeof this.fn[prop] === 'function') {
                    this.fn[prop]();
                }
            }
        }
    };

    Parallax.init();

这是本节的html：

    <div id="site">
      <div id="section-wrapper">
        <div class="section" id="section-1"></div>
        <div class="section" id="section-2"></div>
        <div class="section" id="section-3"></div>
      </div>
    </div>

    <div id="site">
      <div id="section-wrapper">
        <div class="section" id="section-1"></div>
        <div class="section" id="section-2"></div>
        <div class="section" id="section-3"></div>
      </div>
    </div>