4. Startup Delay

Time to Interaction (TTI) is a key aspect of how a user perceives the performance of an app. The TTI of our iOS app is noticeably longer than other competing messaging apps written in native code. Since React Native loads and parses all the Javascript code before execution, you get startup delay based on bundle size. In our case, it’s roughly 1.5s startup delay to load a 15mb bundle on an iPhone X.

Fortunately, Facebook documented unbundling to only load necessary portions of the bundle for startup and progressively load more as needed via their packager.