I highly recommend against using web components. Polymer has failed to take off in the past 3/4 years, and there are performance issues across browsers.
Subjectively, I didn’t find the API all that great either.
Polymer is crap. Web Components are great. There is definitely no performance issues. Ditch polymer and other such bloated libs. Try this 200-line lib instead: https://github.com/wisercoder/uibuilder