Fabric, TurboModules, and fewer bridge surprises. What we had to update in libraries and when we still turned the old arch off.
The New Architecture is not a single switch. It is Fabric for layout, TurboModules for native modules, and a different threading model so your JS and native work don't fight as much.
In greenfield Expo apps, you are usually on it by default. In older repos, the pain is third-party libraries. If a package still assumes the old bridge, you get obscure crashes or "native module not found" until you upgrade or replace it.
Hermes stays on. The wins we saw were smoother lists and fewer "blank frame" moments on navigation-heavy screens. The tradeoff was build time and a longer upgrade path when a dependency lagged behind.
We keep one device on old architecture for a release or two when a client has a huge native dependency tree. Document that decision. Don't let it become permanent.
If you profile, use React Native's built-in tools plus Flipper. Look for unnecessary re-renders on large lists before you blame Fabric. Often the fix is memoization and stable keys, same as before.
“Good systems are mostly boring parts wired together so they don’t fall over on a Tuesday night.”
Jafrix System