Enable PerfMonitor in react-native app

app-with-perfmonitor.js

import PerfMonitor from 'react-native/Libraries/Performance/RCTRenderingPerf';
PerfMonitor.toggle();
PerfMonitor.start();
 
class App extends Component {
  componentDidMount() {
    setTimeout(() => {
      PerfMonitor.stop();
    }, 8000);
  }
  // ...
}