[RoyceFullerton]

One reason:

"...performance of UIWebViews is less than in mobile Safari. This has a lot to do with the absence of the new Nitro JavaScript engine in UIWebViews, apparently for security reasons. I ran some tests on my iPhone 4 with iOS 5.1.1, the JavaScript benchmark Sunspider running in Mobile Safari was 3 x as fast as running in a native app with a UIWebView. Also, to communicate from the UIWebView to the native app, a JavaScript bridge is needed. This is tricky stuff, slow and not really thread safe."

http://blog.mobtest.com/2012/05/heres-why-the-facebook-ios-a...

[raganwald]

This.

IIRC, the reason is that the Nitro engine compiles directly to native code, which is then executed. This requires writing to memory as data and then executing the data. However, Apple doesn’t allow third-party applications to execute data for any reason. This would violate security policies. Thus, any and all third-party scripting—be it JavaScript in a Webkit view or something else—must be interpreted. The best a third-party application can do is compile to byte codes and then interpret the byte codes.