We recently investigated using Xamarin for an app. We utilized the C# code we had already written for the Windows RT version of our app. Some specific details had to be rewritten for the Android version.
What we discovered is that I/O in Xamarin C# is approximately 2x slower than Java. Our app is heavily I/O bound. We have not discovered the cause of this yet, but at the moment we are assuming that it is due to marshaling. While we do try to stay inside the Mono VM most of the time, we do not know how Mono actually accesses the disk.
It is also telling that our C# code uses SQLite.NET (https://github.com/praeclarum/sqlite-net). Identical fetches using the SQLite.NET code are also 2x slower than using Android's Java SQLite wrapper. After looking at the source code, it appears to bind directly to the C .dll, so I do not know why it's so much slower. One possibility is that marshaling strings from native to Java may be faster on Android than native to C# is on Xamarin.
