[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).


const nonExtensibleObject = Object.preventExtensions({});
    const testMap = new Map([[nonExtensibleObject, null]]);
    const testSet = new Set([nonExtensibleObject]);
    
    
    
    testMap.set(0, 0);
    testSet.add(0);
  } catch (e) {
    
    hasBadMapPolyfill = true;
  }
}
export type Fiber = {|
  
  
  
  
  
  
  
  
  
  tag: TypeOfWork,
  
  key: null | string,
  
  type: any,
  
  stateNode: any,