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?
};
export type Thenable = {
  then(resolve: () => mixed, reject?: () => mixed): mixed,
};
const {ReactCurrentOwner} = ReactSharedInternals;
const {
  invokeGuardedCallback,
  hasCaughtError,
  clearCaughtError,
} = ReactErrorUtils;
let didWarnAboutStateTransition;
let didWarnSetStateChildContext;
let warnAboutUpdateOnUnmounted;
let warnAboutInvalidUpdates;
if (__DEV__) {
  didWarnAboutStateTransition = false;
  didWarnSetStateChildContext = false;
  const didWarnStateUpdateForUnmountedComponent = {};
  warnAboutUpdateOnUnmounted = function(fiber: Fiber) {
    
    
    const componentName = getComponentName(fiber.type) || 'ReactClass';
    if (didWarnStateUpdateForUnmountedComponent[componentName]) {
      return;
    }
    warningWithoutStack(
      false,
      "Can't call setState (or forceUpdate) on an unmounted component. This " +
        'is a no-op, but it indicates a memory leak in your application. To ' +
        'fix, cancel all subscriptions and asynchronous tasks in the ' +