onmouseout后图片切换延迟变长，为什么？
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
*{margin:0;
padding:0;
list-style:none;}
.wrap{height:170px;
width:490px;
margin:60px auto;
overflow: hidden;
position: relative;
margin:100px auto;
cursor:pointer;}
.wrap ul{position:absolute;}
.wrap ul li{height:170px;}
.wrap ol{position:absolute;
right:5px;
bottom:10px;}
.wrap ol li{height:20px; width: 20px;
background:#ccc;
border:solid 1px #666;
border-radius:10px;
margin-left:5px;
color:#000;
float:left;
line-height:center;
text-align:center;
cursor:pointer;}
.wrap ol .on{background:#E97305;
color:#fff;}
</style>
<script type="text/javascript">
window.onload=function(){
var wrap=document.getElementById('wrap'),
pic=document.getElementById('pic'),
list=document.getElementById('list').getElementsByTagName('li'),
index=0,
timer=null;
// 定义并调用自动播放函数
function autoPlay(curpic){
clearInterval(timer);
index = curpic;
timer = setInterval(function(){
showPic(index);
select(index);
index++;
if(index>=list.length){
index=0;
}
},2000);
}
autoPlay(0);
// 定义图片切换函数
function showPic(curindex){
pic.style.top = '-'+curindex*170+'px';
}
// 鼠标划过整个容器时停止自动播放
for(var i =0;i<list.length;i++){
list[i].index = i;
list[i].onmouseover = function(){
clearInterval(timer);
select(this.index);
showPic(this.index);
}
list[i].onmouseout = function(){
select(this.index);
autoPlay(this.index);
}
}
function select(current){
for(var j=0;j<list.length;j++){
list[j].className = "";
}
list[current].className="on";
}
wrap.onmouseover = function(){
clearInterval(timer);
}
wrap.onmouseout = function(){
autoPlay(index);
}
// 鼠标离开整个容器时继续播放至下一张
// 遍历所有数字导航实现划过切换至对应的图片
}
</script>
</head>
<body>
<div id='wrap'>
<ul id="pic">
<li><img src="http://img1.sycdn.imooc.com//54111cd9000174cd04900170.jpg" alt=""></li>
<li><img src="http://img1.sycdn.imooc.com//54111dac000118af04900170.jpg" alt=""></li>
<li><img src="http://img1.sycdn.imooc.com//54111d9c0001998204900170.jpg" alt=""></li>
<li><img src="http://img1.sycdn.imooc.com//54111d8a0001f41704900170.jpg" alt=""></li>
<li><img src="http://img1.sycdn.imooc.com//54111d7d00018ba604900170.jpg" alt=""></li>
</ul>
<ol id="list">
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
<li>5</li>
</ol>
</div>
</body>
</html>
