My caffeinated instinct is to say basically "yes I'd do anything in Flutter", I honestly would rather stop coding than go back to anything I've done before (ObjC/Swift/Java/Kotlin with side journeys in C++). It boggles my mind how much of a different job dev is with true hot reload.

More carefully, and dealing with what you're indicating more directly:

There's stuff that we just need every millisecond of performance from.

Generally, Dart's great, I don't notice any difference between iOS / Android standard UI platforms.

But...for example, Flutter's image decoding is actually using "native" code behind the scenes, i.e. calling into C or OS-level APIs or browser APIs as needed on each platform. And there's a Flutter package called "image" that's Dart-native but I abhor because I know it's going to be higher latency than going thru lower-level code. (now I'm wondering how Java does this...I wonder if its JNI...)

Let's do a scenario: I've been contracted to build a bus route app for the local gov't. They want an AR feature. What happens if I choose to build on Flutter, build out the basic features, then get to the AR, and I'm getting 5 fps?"

Solution to that is "plugins" - https://docs.flutter.dev/packages-and-plugins/developing-pac... - the intro to the doc is way out of date, like years. TL;DR is you can drop in C / Swift / Java / whatever easily as needed.

You can get a sense of what that looks like from my package for doing ML inference here: https://github.com/Telosnex/fonnx, specifically https://github.com/Telosnex/fonnx/tree/main/lib/models/minis...: X_native.dart shows us calling into shared C code on every non-mobile platform. On mobile, I have to take a dep on specific packaged & code signed libraries provided by Microsoft for ONNX. Then, I (AI, at this point) writes Swift and Kotlin to call into that library. (Swift: https://github.com/Telosnex/fonnx/blob/main/ios/Classes/OrtM..., Kotlin: https://github.com/Telosnex/fonnx/blob/main/android/src/main...)

This might feel convoluted at first, it did to me, but really, all that's going on is: when things are slow, we write a Dart interface, then for each platform where we want to use native code, provide impls of that interface in native.