Metro style apps in Windows 8 have all the performance benefits of IE10 when showing Web content. In Metro style apps, Web content is always JIT compiled and hardware-accelerated. Other platforms do not provide the same level of performance in apps. For example, Cocoa apps on iOS offer significantly worse JavaScript performance (via the UIWebView control) than the same content running in Safari. These Cocoa apps do not enjoy JIT compilation, and these apps cannot show and use Web content the same way the browser on the system can:

Why is this important?

Many applications embed HTML to provide a richer and always up to date experience for consumers. For example, the developer of a restaurant guide app might want to include a live map showing the locations of the list of restaurants the user is choosing from. If you write an app on iOS, common actions like panning and zooming the map will run twice as slow in an app compared with Safari.
Anyone writing a Metro style app for Windows 8 can easily include Web content in their app. In an HTML or XAML app, just include an <iframe> element or a WebView control to get the full benefit of IE 10 performance. To see a sample HTML app that demonstrates this, check out the “Building Your First Metro Style App Using Javascript” hands-on lab at http://www.buildwindows.com/Labs.

With Metro style apps, it’s easy to integrate many existing Web services seamlessly into your app. It’s also possible to build new services for your app that let you deliver dynamic HTML content without having to update your application.
When you include Web content in your Metro style app, your app gets all the performance benefits of IE10 automatically without any additional or special work. JavaScript code continues to run fast with JIT compilation, and your app will automatically use GPU to accelerate HTML graphics.