The problem you have here is that the Android browser does not use graphics hardware acceleration. This means that the standard tricks that Sencha (and other HTML5 libraries such as jQueryMobile, iScroll etc...) use to provide good scrolling performance, such as CSS 3D transforms to cause a your list to be rendered in a separate layer, which can then be translated in hardware, will not work on Android. Instead, list scroll will be performed entirely in software, which is going to be slow!
The Chrome browser, does however provide GPU acceleration. The Android device is more than capable of delivering a good HTML5 experience, it is simply that the standard browser does not take advantage of GPU hardware yet.
Unless you can force your end users to use Chrome (which I doubt), the only option is to degrade user experience, and deliver a slightly simpler UI for Android users.
For further details, see "IMPROVING THE PERFORMANCE OF YOUR HTML5 APP"
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