RN-Hermes

What is hermes?
Hermes is an open-source JavaScript engine optimized for React Native. For many apps, enabling Hermes will result in improved start-up time (Hermes-powered apps launch faster, thanks to build-time precompilation of JavaScript into efficient bytecode.), Hermes is small in APK size, lean on memory, and starts instantly. It won't weigh your app down.

Initially available with react-native version 0.60.4 and only available for android.

With react-native 0.64, Hermes also runs on iOS.

Android app bundles are supported from React Native 0.62 and up.

By default, you will be using Hermes if you're on the New Architecture which is available with react-native 0.68 onwards. By specifying a value such as true or false you can enable/disable Hermes as you wish.

Starting with React Native 0.69.0, every version of React Native will come with a bundled version of Hermes. We will be building a version of Hermes for you whenever we release a new version of React Native. This will make sure you're consuming a version of Hermes which is fully compatible with the version of React Native you're using.

Enabling Hermes

Android

Edit your android/app/build.gradle file and make the change illustrated below:

project.ext.react = [
      entryFile: "index.js",
-     enableHermes: false  // clean and rebuild if changing
+     enableHermes: true  // clean and rebuild if changing
  ] 

Also, if you're using ProGuard, you will need to add these rules in proguard-rules.pro :

-keep class com.facebook.hermes.unicode.** { *; }
-keep class com.facebook.jni.** { *; }

Next, if you've already built your app at least once, clean the build:

$ cd android && ./gradlew clean

That's it! You should now be able to develop and deploy your app as usual:

$ npx react-native run-android

iOS

To enable Hermes for iOS, edit your ios/Podfile file and make the change illustrated below:

use_react_native!(
     :path => config[:reactNativePath],
     # to enable hermes on iOS, change `false` to `true` and then install pods
     # By default, Hermes is disabled on Old Architecture, and enabled on New Architecture.
     # You can enable/disable it manually by replacing `flags[:hermes_enabled]` with `true` or `false`.
-    :hermes_enabled => flags[:hermes_enabled],
+    :hermes_enabled => true
   )

Once you've configured it, you can install the Hermes pods with:

$ cd ios && pod install

Size of apk (react-native 0.69.1)

With hermes and proguard enable

Without hermes and proguard

More about hermes

https://reactnative.dev/docs/hermes
https://hermesengine.dev/