Only JS execution is 3x slower -- everything else (DOM, layout, repaint, parsing) is the same.
Most web pages (and even web applications) are rarely bottlenecked by JS execution speed. (There are a few exceptions to this rule...for example, the JS GameBoy Color emulator probably won't work very well)
You mean the same as Mobile Safari, not the same as Chrome, right? Because "repaint" is not part of WebKit proper (it's implemented separately in Chrome and Safari, for example). And even "layout" is different code on iOS and in other WebKit ports...
Most web pages (and even web applications) are rarely bottlenecked by JS execution speed. (There are a few exceptions to this rule...for example, the JS GameBoy Color emulator probably won't work very well)