What is the Xamarin platform?
Xamarin's platform promotes code sharing as much as possible. Apps are authored in C# and share on average 75% of code across platforms. The Xamarin.Forms API allows native user interfaces to be built in C# or XAML, which can increase code sharing to 100%.
Xamarin apps are fast
Xamarin apps are compiled to a native binary. Mono, a fully-funtional implementation of the .NET runtime, is bundled with the app to provide access to platform-native APIs. This gives Xamarin apps strong performance in demanding situations, like media and complex data visualization.
*Source: Harry Cheung, Mobile App Performance Redux