This talk explains it in more detail, but the key trade-off is that React Native makes use of native platform widgets whereas Flutter rolls its own widgets. It is an oversimplification, but:
Flutter gains performance both avoiding the overhead of this intermediary layer and also (in production/release mode) by running compiled code (hence Dart).
Neither is likely to ever be as good as writing a native application, but both have the potential to be good enough.
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