<!-- Requirements: please go through this checklist before opening a new issue --> - [x] Review the documentation: https://facebook.github.io/react-native - [x] Search for existing issues: https://github.com/facebook/react-native/issues - [x] Use the latest React Native release: https://github.com/facebook/react-native/releases ## Environment OS: macOS High Sierra 10.13.6 Node: 10.0.0 Yarn: 1.9.4 npm: 6.0.1 Watchman: 4.9.0 Xcode: Xcode 9.4 Build version 9F1027a Android Studio: 3.1 AI-173.4907809 Packages: (wanted => installed) react: 16.3.1 => 16.3.1 react-native: 0.55.4 => 0.55.4 ## Description How to process large process in background thread?