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.

I'm working on an app that's cross-platform on mobile devices. I do builds for macOS as a convenience for my colleagues demoing the app. Pretty much zero effort to add that as a target. I get some benefits out of it too in the form of being able to see how well the UI responds to unusual screen dimensions, running in a resizable window.