Jquery实现缓慢显示下拉菜单

HTML代码：

<div class="navMenu">
        <ul>
            <li class="active"><a href="#">首页</a>
                <ul>
                    <span class="corner"></span>
                    <li><a href="#">JavaScript+</a></li>
                    <li><a href="#">语文</a></li>
                    <li><a href="#">英语</a></li>
                </ul>
            </li>
            <li><a href="#">课程大厅</a></li>
            <li><a href="#">学习中心+</a>
                <ul>
                    <span class="corner"></span>
                    <li><a href="#">JavaScript+</a></li>
                    <li><a href="#">语文</a></li>
                </ul>
            </li>
            <li><a href="#">帮助</a></li>
        </ul>
    </div>

css代码：

 *{
            padding:0;
            margin:0;
        }
        /*一级菜单*/
        .navMenu {
            width:570px;
            margin:0 auto;
            margin-top: 20px;
            position: relative;
        }
        .navMenu ul li{
            float: left;
        }
        li{
            list-style: none;
            background-color: #eee;
            width: 140px;
            height: 34px;
            text-align: center;
            margin-right: 2px;
            margin-bottom: 2px;
        }
        .navMenu ul li:hover{
            background:url(images/slide-pannel_14.png) 0 0 repeat-x;
        }
        ul li a{
            line-height: 34px;
            text-align: center;
            font-size: 20px;
            color: #000;
            text-decoration: none;
            display: block;
            padding:0 10px;
        }
        /*二级菜单*/
        .navMenu ul li ul {
            display: none;
            position:absolute;
            top:34px;
        }
        .active{
            background:url(images/slide-pannel_14.png) 0 0 repeat-x;
        }
        .corner{
            display: block;
            height: 11px;
            background: url("images/bird.png") 61px 0 no-repeat ;
        }

Js代码：

<script type="text/javascript" src="js/jquery-1.12.3.min.js"></script>
    <script type="text/javascript">
        $(document).ready(function(){
            $(".navMenu li").mouseenter(function(){
                $(this).find("ul").slideDown("slow"); //慢慢展开
            }).mouseleave(function(){
                $(this).find("ul").slideUp("slow");//慢慢收起
            })
        })
    </script>