[trevordixon]

Polymer 1.0 uses "Shady DOM" instead of a full shadow DOM polyfill. According to https://www.polymer-project.org/1.0/articles/shadydom.html, "Many projects simply cannot afford the Shadow DOM Polyfill, for the reasons given above. In particular, the performance costs on platforms like mobile Safari are nearly intolerable."

"Polyfilling shadow DOM is hard, the robust polyfill is invasive and slow. Shady DOM is a super-fast shim for shadow DOM that provides tree-scoping, but has drawbacks. Most importantly, one must use the shady DOM APIs to work with scoped trees."

So it sounds like this lighter, less complete Shady DOM half-polyfill that 1.0 uses is specifically made to make performance acceptable on iOS.

[spankalee]

Exactly. Performance under iOS is great with Shady DOM, at the expense of needing to use its APIs to get the effects of DOM scoping and composition.