Flutter btw is quite good for that, at least the app side of things (author was focused on web). I test our flutter app regularly with an old underpowered phone

Flutter btw is quite good for that, at least the app side of things (author was focused on web). I test our flutter app regularly with an old underpowered phone and it always worked well, with performance only getting better with newer versions - probably the project's work against shader stutters helps old devices as well.