If you're talking about the in-client store UI, I believe it's PHP-generated markup rendered in an embedded WebKit browser. Given that the overlay browser uses the same tooling and also renders pages slowly, I'm inclined to believe they're doing something inefficiently, but don't necessarily need to resort to native code to fix.

Native code may be runtime faster, but there's huge advantages to leveraging HTML. You'd have to either -

* Define your own serialization/markup for describing a UI, then specify a network transport for it.

* OR, just bake the whole UI in native code and fill it in with data pulled down from an API. Update the binary when you want to make UI changes.

Neither solution comes with the ease-of-development and flexibility that HTML/HTTP provide.