I've tested this out with trivial list apps as well as the kitchen sink.
Is this the fault of Ext.scroll.Scroller or the browser? Scrolling performance is pretty decent on Android and iOS.
Sidenote: Why does Sencha delegate scrolling behaviour to Ext.scroll.Scroller instead of the device's platform (like in JQuery Mobile)? It seems weird to have elastic scrolling for Android when it's not part of their UI language.