It could also be using something like Ejecta[1], which wraps a javascript engine with a simple native reimplementation of the canvas API. Ejecta is great because its lightweight - you don't pull in all of Webkit like cordova. (And thus you don't have to deal with any of its opaque, ever-changing quirks.)
I'm not an Android or Swift dev, but I imagine you can just stick it in a native webview, which is what I believe Cordova does.
As for performance I can't imagine it's super-performant, but I do know the lead dev does his best to aim for 60fps rendering even under heavy duress.