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?
}
}
let firstScheduledRoot: FiberRoot | null = null;
let lastScheduledRoot: FiberRoot | null = null;
let callbackExpirationTime: ExpirationTime = NoWork;
let callbackID: *;
let isRendering: boolean = false;
let nextFlushedRoot: FiberRoot | null = null;
let nextFlushedExpirationTime: ExpirationTime = NoWork;
let lowestPendingInteractiveExpirationTime: ExpirationTime = NoWork;
let deadlineDidExpire: boolean = false;
let hasUnhandledError: boolean = false;
let unhandledError: mixed | null = null;
let deadline: Deadline | null = null;
let isBatchingUpdates: boolean = false;
let isUnbatchingUpdates: boolean = false;
let isBatchingInteractiveUpdates: boolean = false;
let completedBatches: Array<Batch> | null = null;
let originalStartTimeMs: number = now();
let currentRendererTime: ExpirationTime = msToExpirationTime(
  originalStartTimeMs,
);
let currentSchedulerTime: ExpirationTime = currentRendererTime;
 Use these to prevent an infinite loop of nested updates
const NESTED_UPDATE_LIMIT = 50;
let nestedUpdateCount: number = 0;
let lastCommittedRootDuringThisBatch: FiberRoot | null = null;
const timeHeuristicForUnitOfWork = 1;
function recomputeCurrentRendererTime() {
  const currentTimeMs = now() - originalStartTimeMs;