If a web app is run from the iOS 4.3 home screen - in other words, if it is saved to the screen alongside local apps downloaded from the Apple App Store - it runs roughly two to two and a half times slower than it does in the browser, according to various tests. It appears that whereas Apple has updated the iOS 4.3 Safari browser with its high-speed Nitro JavaScript engine, Nitro is not used when web apps are launched from the home screen.
"Essentially, there are two different JavaScript engines," says Alex Kessinger, a mobile application developer and blogger who has focused on building web-standards-based apps for the iPhone. "They're not using the new JavaScript engine with applications that launch from the home screen."