Master JavaScript Performance in React Native

Szymon connects delays in app startup and response time to lost users and revenue. He says the session will focus on JavaScript performance, where teams can have a large impact and often make avoidable mistakes.

A context provider that creates a new value object on every render can trigger renders across its consumers, even when the values are unchanged. Szymon recommends memoizing the value with `useMemo`; in one audited app, he found this issue six times. For FlatList, an inline `renderItem` changes identity on each parent render, so wrapping it in `useCallback` can avoid unnecessary item renders.

JavaScript runs one task at a time, so expensive work can delay user interactions. In a date-formatting example, recreating `DateTimeFormat` for each item took about 20 to 23 milliseconds, while creating it once and reusing it took about 5 milliseconds. Szymon advises profiling first and applying memoization where measurements show a meaningful benefit.

A workflow editor became sluggish because changing one node triggered synchronous validation of the whole workflow, causing about half a second of delay between keystrokes on some devices. Szymon demonstrates splitting divisible work into smaller tasks and yielding between them so JavaScript can handle user events. Wrapping synchronous computation in a promise does not make that computation run in the background.