Hmm, so if you wanted to make an AR app, or some audio processing app, would you do that in Flutter? All the projects I have in mind involve using the camera/microphone/gps etc. Looking at Dart sample projects it just seemed to be quite different from what they're aiming at

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 - the intro to the doc is way out of date, like years.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.