With the big caveat that only JavaScriptCore can JIT, so third-party interpreters will always be performance-constrained. Not a big deal for languages like Python or Lua that are always interpreted, but things like .NET on iOS (Xamarin) have to be ahead-of-time compiled for decent performance, and it's a complete non-starter for third-party JavaScript interpreters.
JavaScriptCore can’t JIT when run in process, the entitlement isn’t there on your app, so the kernel won’t let it. I think that’s part of the reason for WKWebView (which runs the browser in a separate process and proxies it).
