I wonder how much faster phones need to be for it not to matter (i.e. native is still faster, but not noticeably). Would dual core be enough? Assuming the engines can scale behind the scenes for us, how many cores would be needed?

This would change everything, especially for the iPhone's walled gargen (unless new apps appear that require the extra power - perhaps console-quality games? complex visualizations)

Intel managed an amazing trick, of keeping demand for more processing power strong for decades (from 1978 to 1998; when the celeron was introduced, for the market that valued low price over performance; and 2008, the Atom for low power consumption over performance). But to really compete against ARM requires letting go of the blessing and the curse of back-compatibility - which Intel hasn't succeeded at so far (see Itanium). The demand for performance seems to be ending, and I expect with it, Intel will also - perhaps by 2018.

My point was more that PhoneGap + jQuery Mobile allows you to get something out really quickly which can then be replaced by a native app as time or money allows. In my startup's case we're using this approach as none of us are proficient in Obj C so while we catch up with that and iterate our native app, our designer/front end guy can pretty much build our WebView app himself. In a small team with lots to do this is proving very helpful