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Not an ideal solution
This may not be an ideal solution, but it seems Phonegap offers functionality to add a splash screen. Perhaps you could try making your splash screen display via that instead.
Webview starting up?
I always thought that the few seconds of white screen after launching your natively built sencha app on ios or android is due to webview starting up, and that this was entirely unavoidable. Once that is loaded, then it'll start running your app (including splash screen etc).
The only solution was to compile your app with phonegap which can handle launching your app with a splash screen right away after pressing your app's icon (looks more professional).