I can't speak to load times since I haven't bench marked it against an identical app in React or something; but I think Polymer has some clear uses. If you want to create a widget that will be slightly configurable and easily embedded on another page (the big Polymer example I remember is Google Maps), then the isolation web components provide is a good option. If you're just looking to create a standalone app I'm not sure if it's worth breaking you app up into independent chunks.