Loading this snippet of HTML in content shell takes 15s on my workstation.

<div id="e">
</div>
<script>
  let start = Date.now();
  console.log("here");
  for (i=0; i< 200; i++) {
      e.innerText += "AXRole: AXColumnHeader col head\n";
  }
  console.log("here");
  console.log(Date.now()-start);
</script>

- Removing the \n drops it to 4s which is still very slow.
- Switching to e.textContent makes it take 38ms which seems much closer to what I would expect.

yosin@ tells me that getting .innerText causes a full layout but thinks maybe we really only need to recalc style.

I dug into this because of  http://crbug.com/898049  .