Unfortunately there's still a glaring bug. The new WKWebView (webview with optimized JavaScript using Nitro engine) can't load html docs from 'file://' (local file system). Also, it's my understanding that the old UIWebView performance degraded significantly for html/JS. So this really negatively affects Cordova/Phonegap apps.

Note: I haven't verified any of these claims, but have been reading up on it and have an active interest in the situation since I write Cordova/Phonegap apps.