 <p><a href="http://xamarin.com/monotouch">Monotouch</a> is not a cross-compiler, and even if it were that alone would not cause a performance penalty.</p> <p>Monotouch is a .Net compatible runtime that <a href="http://www.mono-project.com/Mono%3aRuntime#Embedded_systems">runs on iOS embedded</a> within a normal c/obj-c program. It includes bindings for more or less the same things you can do from normal iOS apps. Monotouch apps are AOT compiled rather than JIT compiled so in most cases they run just as fast as "normal apps"</p> <p>The fact that <a href="http://www.sencha.com/products/touch">SenchaTouch</a> is a javascript framework suggests to me that you will be running mainly interpretted code. I'm sure the Sencha people have done a good job but I would be suprised if it were faster than mono. Looking at Sencha, one advantage is that you can write for android, blackberry and iOS. You can share much of your code between MonoTouch and <a href="http://xamarin.com/monoforandroid">Mono for Android</a>, you need two licenses and can't share everything.</p>