I finally moved to Xamarin a few months ago and it's not slower than the native languages. No clunkiness either, it's just bindings to the native APIs after all.
Sorry, I was not referring to the execution speed of c# once deployed, but the experience of pressing F5 to debug the app in an emulated environment is pretty slow and hairy, and makes development hard.
I don't see much difference in the iOS simulator: deploy and launch take 3-5s. It's about the same for the Android emulator but I have no experience with Java development, so I don't know if it's fast enough.
I am much less of a fan of Xamarin, tried it, too clunky (for instance the default settings for the android emulator do not work out of the box) and slow.
And it’s a matter of taste. I don’t like dynamic languages as I’d rather have the compiler check my syntax for me rather than having to write a test, and static typing enables great refactorings. But other people will have other priorities.