Earlier quoted context omitted.
It's not only WebKit, but also the Javascript engine and to make matters worse, on iOS last time I checked the web view that you could use in apps wasn't using the same Javascript engine that Safari was using, which means that an alternative browser is slower than Safari by default. From what I've been hearing, lately developers can use the same engine as Safari inside those web views, but the functionality is still…
Apparently on iOS 8, Apple finally ended the anti-competitive restriction that made all Safari skin apps (aka "browsers" on iOS) slower than Safari itself.
All this talk of anti-competitive restrictions doesn't reflect the reality of engineering for security.