The lack of the faster Nitro javascript engine in webviews on iOS is probably a major reason for this.
> The Nitro JavaScript engine is only available within Mobile Safari. Outside Mobile Safari — whether in App Store apps using the UIWebView control, or in true web apps that have been saved to the home screen — apps get iOS’s older JavaScript engine. [1]
> “Apple is basically using subtle defects to make web apps appear to be low quality — even when they claim HTML5 is a fully supported platform,” says one mobile web app developer, who asked that his name not be used.[2]
> The Nitro JavaScript engine is only available within Mobile Safari. Outside Mobile Safari — whether in App Store apps using the UIWebView control, or in true web apps that have been saved to the home screen — apps get iOS’s older JavaScript engine. [1]
> “Apple is basically using subtle defects to make web apps appear to be low quality — even when they claim HTML5 is a fully supported platform,” says one mobile web app developer, who asked that his name not be used.[2]
[1][2] http://daringfireball.net/2011/03/nitro_ios_43
AFAIK all that is still true today aside from webapps saved to the home screen (which now does use the Nitro engine)