# Ship fewer JS threads — a React Native interaction budget

Users forgive a slow cold start more than a laggy tap. After the first screen, every interaction is a promise: press  feedback within ~100ms, content within a few hundred.

## Pick one number

On a mid-range Android (or an emulator profile that matches one), time **tap → first paint of the next UI**. Do it for your worst navigation: open a heavy list, open a modal with a form, switch tabs that each mount a stack.

Write the number down. That is the interaction budget. Everything else is negotiation.

## Three buckets (again)

Same mental model as startup, different owners:

1. **JS work on the critical path** — parsing, sorting, Redux selectors, big JSON.
2. **Bridge / native layout** — too many shadow-tree updates, measure passes.
3. **List & image cost** — mounting 200 rows, decoding huge bitmaps on the UI thread.

You do not need perfect attribution. You need to know which bucket owns most of the delay on *your* worst path.

## Cheap wins that usually move the needle

- **Defer non-visible work** with `InteractionManager.runAfterInteractions` (or a microtask queue you control) so the press animation and route transition finish first.
- **Don't block the press handler** on network. Optimistic UI or a skeleton beats a frozen button.
- **Lists:** fixed `getItemLayout` when you can, smaller row components, avoid anonymous inline functions that defeat memo, and prefer a virtualized list that matches your RN version (FlashList when it fits).
- **Hermes + production minify** for release measurements; Debug JS is a different sport.
- **Images:** correct size, caching, and avoid decoding 4k assets for a 96px avatar.

## What I ignore early

Micro-optimizing re-renders of a leaf label while a 2MB JSON parse sits on the press path. Flamecharts lie less than gut feel — capture one interaction, fix the fattest frame, remeasure.

## Done looks like

Same mid-range device, same path, a lower number you would show a PM. If it did not move, you optimized the wrong bucket — pick another and try again.

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Read the canonical version: https://www.ashishkumar.dev/posts/ship-fewer-js-threads/