BUNDLE and MAP process for index.js is taking a long time

Start by reproducing the delay with yarn android and yarn start, then inspect the attached React Native DevTools trace for work blocking the blank screen while index.js is bundled and mapped. Compare behavior on the reported React Native versions and document a reproducible cause or actionable next step; the issue currently names no project files or tests.

Description

I recently upgraded my application's React Native version from 0.80.2 to 0.86.0 (the latest stable release). However, since then, the bundle and map process for the application's index.js file has become extremely slow; the app always stays on a blank screen with no UI response for over 100 seconds while the thread is running. I’ve tried various workarounds—such as initializing libraries asynchronously and using requestNextFrame to prioritize the UI rendering before any heavy processing takes place—but nothing has worked so far. Does anyone have any ideas or suggestions as to what exactly might be causing this?

I’ve shared a performance trace generated by React Native DevTools below; the screen was blank throughout the entire trace:

Trace-20260916T122443.json

Steps to reproduce

1. Execute yarn android
2. Execute yarn start in another terminal
3. Open app

React Native Version

0.86.0

Output of npx @react-native-community/cli info

System:
  OS: Linux 7.0 Ubuntu 24.04.5 LTS 24.04.5 LTS (Noble Numbat)
  CPU: (12) x64 AMD Ryzen 5 5600G with Radeon Graphics
  Memory: 2.78 GB / 14.99 GB
  Shell:
    version: "5.9"
    path: /usr/bin/zsh
Binaries:
  Node:
    version: 26.2.0
    path: /home/sandrords/.nvm/versions/node/v26.2.0/bin/node
  Yarn:
    version: 1.22.22
    path: /home/sandrords/.nvm/versions/node/v26.2.0/bin/yarn
  npm:
    version: 11.13.0
    path: /home/sandrords/.nvm/versions/node/v26.2.0/bin/npm
  Watchman: Not Found
SDKs:
  Android SDK:
    API Levels:
      - "33"
      - "34"
      - "35"
      - "36"
    Build Tools:
      - 30.0.3
      - 33.0.0
      - 34.0.0
      - 35.0.0
      - 36.0.0
    Android NDK: Not Found
IDEs:
  Android Studio: Not Found
Languages:
  Java:
    version: 17.0.20
    path: /usr/bin/javac
  Ruby:
    version: 3.2.2
    path: /home/sandrords/.rbenv/shims/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 20.2.0
    wanted: ^20.1.0
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.86.2
    wanted: ^0.86.0
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: true
  newArchEnabled: false

Screenshots and Videos

No response