There are two problems. The first problem is political - firstly Apple make it so that you have to use the Webkit rendering engine for any third party browser.

There are two problems. The first problem is political - firstly Apple make it so that you have to use the Webkit rendering engine for any third party browser. The second problem is more technical - iOS has memory protection that means you can't take a writeable page and make it executable. This means that you can't (for example) JIT compile Javascript (or any other language). Their built in exception to this is Safari. This is why Safari is faster than webviews.