This is why Mobile Safari is quicker than google chrome for iOS.
Mobile Safari, the default browser in iOS, uses a Javascript engine called Nitro, which in turn uses a technology called "just-in-time" compilation, or JIT, to execute scripts more quickly. JIT makes Nitro faster, and Nitro makes Mobile Safari faster. This is great, if you're using Mobile Safari.
But other apps that want to include a browser function, be they Facebook or an actual alternative browser like Chrome, don't get Nitro. And developers can't use their own engines, either. For security reasons, the browser developers get to use in their apps is a variant of an older, pre-Nitro version, called UIWebView.