At least on Android, it "feels" fast enough. However, it breaks navigation when you use the Back button (in the interface) and the Back button (part of Android).
Not if you use routes properly. I've been able to get the browser back and device back button on android work right for my work.
On "feeling fast enough", let's pick a jquery mobile case, I tried http://www.jqmgallery.com/2012/02/11/sharksfrenzy/ on galaxy s2 (dual core, etc) and iPhone 4s. The list (notice its translucent, and overly decorated) scrolls like snail on the android phone. While it flows very smoothly on iPhone 4s.
A similar list would do better than that on Android if done in Sencha Touch 2.
I don't experience the sluggishness on my Galaxy Nexus in Chrome Mobile Beta (16.0.912.77). It scrolls very smooth (no lag, or jitter), although the refresh button is a little off (square corners on the bottom).
This alone, makes the app clunky.