[This example](https://codesandbox.io/s/1zn2o513pq) presents a huge performance drop between React 16.2 and React 16.3.

In investigated it and noticed several things:

- Perfomance drop is only visible using development bundle
- This is not related to styled-components (a simple div presents the same problem)
- I tried to investigate in performance tab of Chrome dev tools and I noticed that I get React measures even without "?react_perf"
- React Tree Reconciliation seems very long, do not know why

It is not a big problem for me, but it looks weird, so I decided to [share it on Twitter](https://twitter.com/neoziro/status/979756579296817154),  then @gaearon [invited me to submit an issue about it](https://twitter.com/dan_abramov/status/980036013518139398).


'fix, cancel all subscriptions and asynchronous tasks in the ' +
        'componentWillUnmount method.%s',
      getStackAddendumByWorkInProgressFiber(fiber),
    );
    didWarnStateUpdateForUnmountedComponent[componentName] = true;
  };
  warnAboutInvalidUpdates = function(instance: React$Component<any>) {
    switch (ReactDebugCurrentFiber.phase) {
      case 'getChildContext':
        if (didWarnSetStateChildContext) {
          return;
        }
        warning(
          false,
          'setState(...): Cannot call setState() inside getChildContext()',
        );
        didWarnSetStateChildContext = true;
        break;
      case 'render':
        if (didWarnAboutStateTransition) {
          return;
        }
        warning(
          false,
          'Cannot update during an existing state transition (such as within ' +
            "`render` or another component's constructor). Render methods should " +
            'be a pure function of props and state; constructor side-effects are ' +
            'an anti-pattern, but can be moved to `componentWillMount`.',
        );
        didWarnAboutStateTransition = true;