Guides
When to use a second runtime
A decision guide for choosing between main, threaded UI, and a background runtime.
React Native Runtimes gives you three places to put work:
| Place | What it owns |
|---|---|
| Main runtime | Navigation, gestures, top-level state, animations |
| Threaded UI runtime | A whole screen or component that re-renders often |
| Background runtime | App-lifetime work with no UI: sync, caches, queues |
The right place depends on whether the work is visible, whether it blocks, and whether it's shared across screens.
Decision flow
- Does the work render UI?
- No → use a background runtime
- Yes → continue
- Does the rendering ever stutter, drop frames, or block input?
- No → keep it on the main runtime
- Yes → continue
- Is the rendering self-contained (one screen, one big list)?
- Yes → push it to a threaded UI runtime
- No → reshape it first (memoize, virtualize, batch)
When the main runtime is correct
- Top-level navigators
- Simple screens that don't render expensive lists
- One-off forms, settings, small dialogs
- Anything that needs synchronous access to navigation state
When a threaded UI runtime helps
- A chat screen with a virtualized list of hundreds of messages
- A feed with image-heavy items
- A document or PDF reader
- A code editor or markdown preview
- Any list using FlashList / LegendList at scale
A useful question to ask
"If this screen alone caused 50 ms of JS work per frame, what else in the app would suffer?" If the answer includes navigation or core gestures, that screen belongs on its own runtime.
When a background runtime helps
- A sync engine that hits the network on its own schedule
- A queue of pending jobs (retry, dedupe, persistence)
- Cache hydration on cold start
- Document parsing or local search indexing
- Crypto orchestration through native modules
When not to use a second runtime
- The screen already runs smoothly on the main runtime.
- The work is one-off (it doesn't dominate any frame).
- The runtime cost would outweigh the gain — e.g. a trivial component.
- The work depends heavily on main-runtime singletons (navigation refs, refs from React).