It depends on what you measure. Google argues that "time to interactive" is the correct number to optimize for; that's where Polymer-based sites shine. But nob

It depends on what you measure. Google argues that "time to interactive" is the correct number to optimize for; that's where Polymer-based sites shine. But nobody can beat streaming server-rendered (or ServiceWorker-rendered) HTML + CSS on a "time to paint" basis. Google claims that "time to paint" is the wrong measure, but if you're using progressive enhancement, TTP is all you need. Shadow DOM mostly sucks anyway for lack of a good way of sharing styles between shadow roots. (It's awesome that we can encapsulate CSS with shadow DOM, but that was too much encapsulation.)