Keep in mind that the performance here is that of mobile WebView components, not PhoneGap particularly. Mainly PG just creates a shell app that creates a WebView and then runs your content in it. So whether your content would perform better today than two years ago is a matter of whether iOS and Android have improved their WebViews[0]. PhoneGap, for its part, has added more platforms and gotten a much improved extension system in the last two years, but nothing I know of that would seriously affect performance.
[0] Android WebViews seem to improve steadily, while always lagging a bit behind the real browser. iOS WebViews were crappy for a long time, particularly because Apple kept Nitro for themselves while third-party hybrid apps used a way slower JS engine. But iOS 8 apparently made massive strides here, and WebViews are right on par with mobile Safari (Nitro, WebGL, etc). Or so they say; I haven't tried it out yet.
[0] Android WebViews seem to improve steadily, while always lagging a bit behind the real browser. iOS WebViews were crappy for a long time, particularly because Apple kept Nitro for themselves while third-party hybrid apps used a way slower JS engine. But iOS 8 apparently made massive strides here, and WebViews are right on par with mobile Safari (Nitro, WebGL, etc). Or so they say; I haven't tried it out yet.