[Bug Report] el-tree checkbox is caton

### Element UI version
2.6.3

### OS/Browsers version
win7/chrome

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/superchangme/28w37auc/

### Steps to reproduce
1.使用谷歌浏览器打开链接
2.点击页面中的添加树按钮
3.选中某个节点发现大概卡了1s左右（一共有2600多条节点数据）

### What is Expected?
选中节点不卡顿

### What is actually happening?
选中节点卡顿1s