fwiw if you read the rest of that thread on Twitter it explains that Youtube is not using Shadow DOM v0, the slowness is likely related to polyfills for HTML Im

fwiw if you read the rest of that thread on Twitter it explains that Youtube is not using Shadow DOM v0, the slowness is likely related to polyfills for HTML Imports, and the next version of Polymer no longer uses those to resolve issues like this. Also Chrome is actively removing the web standards that didn't end up being adopted broadly: https://groups.google.com/a/chromium.org/forum/m/#!topic/blink-dev/h-JwMiPUnuU https://groups.google.com/a/chromium.org/forum/m/#!topic/bli... So YouTube will end up just as slow in other browsers unless they upgrade to a newer polymer that uses the broadly implemented standards.