In my experience performance is about the same as native on mobile. On desktop I cannot compare as I thankfully never had to make cross platform desktop apps using native platform SDKs, but Flutter is doing fine. I am a working on a non trivial desktop app, and I am pretty happy about it.
Hopefully the desktop story is going improve as Canonical is now leading the Flutter desktop side.
Hopefully the desktop story is going improve as Canonical is now leading the Flutter desktop side.