Investigate performance issues with Session Replay on iOS

[aminerol]

What React Native libraries do you use?

React Navigation, Hermes, Expo Application Services (EAS), Expo (mobile only)

Are you using sentry.io or on-premise?

sentry.io (SaS)

@sentry/react-native SDK Version

6.19.0

How does your development environment look like?

System:
  OS: macOS 14.7.1
  CPU: (14) arm64 Apple M3 Max
  Memory: 1.57 GB / 36.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 22.17.1
    path: ~/.nvm/versions/node/v22.17.1/bin/node
  Yarn:
    version: 1.22.22
    path: /opt/homebrew/bin/yarn
  npm:
    version: 10.9.2
    path: ~/.nvm/versions/node/v22.17.1/bin/npm
  Watchman:
    version: 2025.03.10.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.16.2
    path: /opt/homebrew/bin/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 24.2
      - iOS 18.2
      - macOS 15.2
      - tvOS 18.2
      - visionOS 2.2
      - watchOS 11.2
  Android SDK: Not Found
IDEs:
  Android Studio: 2024.1 AI-241.18034.62.2411.12071903
  Xcode:
    version: 16.2/16C5032a
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.12
    path: /usr/bin/javac
  Ruby:
    version: 2.6.10
    path: /usr/bin/ruby
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 18.2.0
    wanted: 18.2.0
  react-native:
    installed: 0.74.2
    wanted: 0.74.2
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: false
iOS:
  hermesEnabled: true
  newArchEnabled: fals

Sentry.init()

Sentry.init({
    dsn: 'https://...@sentry.io/...'
    // other options
    tracesSampleRate: 1.0,
    profilesSampleRate: 1.0,
    replaysOnErrorSampleRate: 1.0,
    replaysSessionSampleRate: 1.0,
    integrations: [
      reactNativeTracingIntegration(),
      mobileReplayIntegration({
              maskAllText: false,
              maskAllImages: false,
              maskAllVectors: false,
              enableViewRendererV2: true,
              enableFastViewRendering: true,
       }),
      hermesProfilingIntegration(),
      navigationIntegration,
    ],
    enableAutoPerformanceTracing: true,
    attachViewHierarchy: true,
    attachScreenshot: true,
    enableAppStartTracking: true,
    enableNativeFramesTracking: true,
    enableStallTracking: true,
    enableUserInteractionTracing: true,
    enabled: !__DEV__,
    ignoreErrors: [/Amplitude Logger/i],
});

Steps to Reproduce

Nothing much to reproduce it, just open the app and once you get replay on sentry dashboard they are all black for iOS

Expected Result

an actual video of session replay on sentry dashboard

Actual Result

[linear]

RN-304 Black session replays on iOS

[kahest]

Hey @aminerol - a few questions: does it work on Android? Did you try and reproduce this on simulator and on real device?

Also can you try again after turning enableFastViewRendering off? This is a slightly more aggressive mode that may impact Replays and may be removed in the future.

[aminerol]

Hey @kahest, this works just fine on android, and this was tested on real device with production bundle using testflight.

and yes the black replays starts happening only after enabling enableFastViewRendering to help with frame drops cause in our case all list stutters when scrolling if session replay is on

[kahest]

There's two settings related to Session Replay render performance on iOS:

• enableViewRendererV2 which is active by default since RN SDK 6.14.0 and usually has a much bigger (positive) perf impact and no known drawbacks
• enableFastViewRendering which is deactivated by default, has a smaller perf impact and known issues (like the one you experience) - this is also why we don't mention this setting in our onboarding docs

So in case you didn't test your app with just the first setting active, I think it'd be worth it. The second option is really a fall back that can work nicely for some apps, but might not work at all for others.

[aminerol]

Thanks for clarifying the differences between the two settings.

We initially enabled enableViewRendererV2, but unfortunately it didn’t resolve the frame drops we were seeing especially the visible jumps during scrolling. That’s why we ended up turning on enableFastViewRendering, which did help with the scrolling issue, but as you mentioned, it comes with drawbacks on iOS Session Replay.

Right now we’re stuck between two trade-offs:
Keep enableFastViewRendering off → smoother scrolling but no usable Session Replay on iOS.
Keep it on → sacrifice scrolling smoothness for the sake of having Session Replay.

Given this situation, are there any workarounds or upcoming improvements planned that could help us achieve both smooth scrolling and reliable Session Replay on iOS?