Why 120 FPS is disabled by default in earlier react-native versions?

[hazemelshaterr]

Hey guys,

There's something I wanted to ask about regarding the 120 FPS animations:

https://docs.swmansion.com/react-native-reanimated/docs/guides/performance/#-enable-120-fps

I noticed that this seems to be enabled by default on iOS starting from react-native 0.82

Is there a reason why it wasn't enabled by default on earlier react-native versions? Were there any known issues, performance concerns, or compatibility problems?

my main question is: if I'm using the old Architecture and CADisableMinimumFrameDurationOnPhone is currently false, is there any risk in enabling it on earlier react-native versions, or should it generally be safe?

Just trying to understand if there are any defects or weird behavior before rolling it out to production.

thanks

[MILLERMARRU]

To answer the concrete part first: yes, CADisableMinimumFrameDurationOnPhone is the same Info.plist flag either way, it's not gated by New or Old Architecture, it just tells iOS to stop capping the frame duration on ProMotion phones. The reanimated docs confirm it's now enabled by default starting React Native 0.82, which is the "why does it suddenly work" part of what you're seeing, earlier RN versions require setting it manually, which is exactly what you're doing.

On whether flipping it on manually for an older RN version is safe: I don't have a documented answer for why it specifically stayed opt-in before 0.82 (the changelog doesn't spell out a "known issue" that gated it, just that the default flipped at that version), so I'd be guessing if I claimed a specific regression it was protecting against. What is documented is that the New Architecture currently has its own separate FPS regressions under load (frame drops while scrolling with many animated components, and lower FPS running multiple simultaneous animations), with dedicated flags to mitigate those (USE_COMMIT_HOOK_ONLY_FOR_REACT_COMMITS, enableCppPropsIteratorSetter, ANDROID_SYNCHRONOUSLY_UPDATE_UI_PROPS/IOS_SYNCHRONOUSLY_UPDATE_UI_PROPS). Those are a separate concern from the 120fps flag itself, but worth knowing about since running at 120fps makes any per-frame overhead twice as visible as it would be at 60fps, if you're going to test this on an older RN version, that's where I'd actually watch for regressions rather than the flag itself misbehaving.