Slow UI or slow loading? Slow loading is due to all the HTML imports generating HTTP requests. This can be optimised by inlining the web-components, using a tool like https://github.com/Polymer/vulcanize

I'm not sure why the polymer website is not optimised like this.

[edit]

Here's what vulcanize does: http://polymer-twitter.badsyntax.co:8002/build.html (Code here: https://github.com/badsyntax/polymer-twitter)

Concatting styles is a whole different story, conceptually it doesn't fit in with encapsulated compontents.