[Bug Report] el-table would rendered three time in every update, so it's slow

[XGHeaven]

Element UI version

1.4.1

OS/Browsers version

Macos

Vue version

2.4.2

Reproduction Link

https://jsfiddle.net/XGHeaven/qwnm2xbq/

Steps to reproduce

use formatter to see how times table render runs.
please see reproduction link.
you can see 12 Called text under table. because there have four rows of table, and one row format(render) three times.

What is Expected?

I expect only call one time of render function.

What is actually happening?

And now, three times called of render function.
Whatever the first render or update render, it always render three times.

[dongsuju]

I also encountered this problem.

[hkongm]

同样碰到这个问题了
the same!