In my experiments, canvas performance on iOS devices is horrible, and especially with Phonegap... it takes from 100..200 ms to draw a single frame, crazy... Try opening iPad http://funwithhtml5.com/source/reportCanvasBug.html and try to rotate iPad and see how from 1..2 ms per frame it goes to 200ms per frame.

I am playing right now with Ejecta which translates JavaScript to ObjectiveC, and then I get a very good speed, no matter how I rotate the screen.