[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).


invariant(
      !isRendering,
      'flushSync was called from inside a lifecycle method. It cannot be ' +
        'called when React is already rendering.',
    );
    const previousIsBatchingUpdates = isBatchingUpdates;
    isBatchingUpdates = true;
    try {
      return syncUpdates(fn, a);
    } finally {
      isBatchingUpdates = previousIsBatchingUpdates;
      performSyncWork();
    }
  }
  function interactiveUpdates<A, B, R>(fn: (A, B) => R, a: A, b: B): R {
    if (isBatchingInteractiveUpdates) {
      return fn(a, b);
    }
    
    
    
    
    if (