The AgilIQ blog has an interesting story where they dissected and analyzed exactly what goes on under the covers between a PhoneGap HTML app and the native device. It’s not too pretty. While the iOS communication stream isn’t too bad, the back and forth messaging that happens in the Android implementation creates a heavy overhead. These observations explain why there are frequent complaints about PhoneGap apps on Android being not too snappy.
Attempts have been made to develop CSS themes that mimic the look of a native app running on each of the different possible target platforms. Often in order to get the theme you need to load up on another framework that supports the theme, but adding another framework to the mix can further impact performance. Trying to create pixel-perfect screens that match up with native ones can, for example, can also involve complicated CSS renderings that draw on the machine’s processing power and further affect performance.