# Workers and performance

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## Engine execution

The default `engineMode="main-thread"` evaluates the form engine on the UI thread. Use
`engineMode="worker"` to move engine evaluation into the packaged module worker when Web Workers
are available:

```jsx
<FormRenderer schema={schema} engineMode="worker" />
```

Vite supports the static module-worker pattern used by the package and emits the worker during a
production build. The maintained Vite starter excludes `form0-react` from dependency optimization
so this worker remains available.

The public `FormRenderer` API does not accept a custom worker URL or worker factory. Applications
using another bundler should confirm that it supports
`new Worker(new URL("./worker.js", import.meta.url), { type: "module" })`; otherwise use the default
main-thread mode.

If the browser has no Worker support, the binding falls back to main-thread execution.

## React store mode

`engineStoreMode="snapshot"` clones the engine snapshot into React state and is the default.
`engineStoreMode="selector"` uses field subscriptions to reduce broad rerenders.

Treat selector mode as an optimization: verify field errors, navigation summaries, repeatable
sections, and submission behavior in the target application before adopting it.