I was looking at https://github.com/facebook/react/issues/12700, and this seemed suspicious.

Consider this case:

```js
class App extends React.Component {
  componentDidMount() {
    for (let i = 0; i < 10000; i++) {
      ReactDOM.render(<div />, document.createElement('div'));
    }   
  }
  render() {
    return null
  }
}

ReactDOM.render(<App />, document.getElementById('root'));
```

Profiling tells me we spend a *lot* of time searching for a root:

<img width="838" alt="screen shot 2018-08-07 at 12 12 52 am" src="https://user-images.githubusercontent.com/810438/43745499-f8716922-99d6-11e8-8822-380dcb15bfa3.png">

Adding some logging shows that e.g. for a 1000 roots, we'll run [this loop iteration](https://github.com/facebook/react/blob/master/packages/react-reconciler/src/ReactFiberScheduler.js#L1798-L1846) 1000 times for the first root, then 999 times for the second root, then 998 times for the third root, etc. Oops.

This is surely a bug?
/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */
import type {Fiber} from './ReactFiber';
import type {FiberRoot, Batch} from './ReactFiberRoot';
import type {ExpirationTime} from './ReactFiberExpirationTime';
import ReactErrorUtils from 'shared/ReactErrorUtils';
import ReactSharedInternals from 'shared/ReactSharedInternals';
import ReactStrictModeWarnings from './ReactStrictModeWarnings';
import {
  NoEffect,
  PerformedWork,
  Placement,
  Update,
  Snapshot,
  PlacementAndUpdate,
  Deletion,
  ContentReset,
  Callback,
  DidCapture,
  Ref,
  Incomplete,