Basically not possible to do JIT (needed for decent JS perf) on iOS for 3p apps (since that requires pages that are both writable and executable) and then on top of that Apple bans 3p browsers specifically in markets where they're allowed to.

When submitting apps to Apple, all browser apps are required to and are basically glorified WKWebViews. Apple doesn't allow any other browser engines when submitting apps to app Store.