I noticed stuttering in my application when using Xamarin.Forms, and i must say GUI performance is very critical to my project. I narrowed down the problem by creating a simple application (see attachment) that creates timer with CreateRepeatingScheduledTimer that randomly updates property value, which is bound to Label text via INotifyPropertyChanged. I made timer callback a bit heavy with a loop inside, and as a result i see that Label to which property is bound is not updated at all, unless i tap on the screen. And this happens in both iPhone and Simulator.
Here is video that shows this behavior: http://www.screencast.com/t/tpwJmbvlW