A recent Daring Fireball article alludes to a iOS 4.3 Mobile Safari vs. UIWebView difference: "[Safari] uses asynchronous multithreading (UIWebView does not)." John Gruber doesn't make it clear what he means by "asynchronous multithreading", and the term itself seems fishy (doesn't threading imply asynchronous behavior? If you're going to block, why bother with threads?).

I tried to trace the source of this, and saw more possibly related references to asynchronous something or others: The Register says: "[UIWebViews] aren't rendered using Apple's newer 'asynchronous mode'. They're saddled with the old 'synchronous mode', which means means they don't quite look as good." Meanwhile, Ars Technica reports: "Developers have also noticed that full-screen Web apps also don't take advantage of MobileSafari's ability to asynchronously load scripts, which can cause some performance issues—particularly for games. The underlying WebKit engine gained this ability late last year, so it's not entirely clear if this issue is a regression, or if it is just new to MobileSafari and hasn't yet been carried over to WebSheet.app."

Based on the Ars Technica article, we would appear to have at last something concrete to test. However, using the <script defer> test case (mentioned in the WebKit blog post on asynchronous loading) with iOS 4.3, not even Mobile Safari actually loads the script asynchronously, (results - with defer support the result should be around 1,000ms). The same thing with a <script async> test case (results).

It's not clear whether Daring Fireball, The Register, or Ars Technica are talking about the same thing. The Register says they talked to an "unnamed developer", and the Ars Technica article came after (and references) The Register one, so it could just be a game of telephone, where they heard "asynchronous" and assumed it was about script loading. If anyone has concrete (technical) details, they would be appreciated. Alternatively, the iOS 4.3 release of WebCore and JavaScriptCore will show up on Apple's Open Source site eventually, and then it might be possible to investigate this behavior directly.