> fine but this is entirely irrelevant. the problem is that no one was optimizing for the deliverable (JavaScript). the GWT folks could have easily written an excellent compiler so it didnt generate a bunch of slow trash.

Compilers can only optimize so much. GWT/Closure do a very good job of it, too. https://www.peterbe.com/plog/advanced-closure-compiler-vs-ug...

Much of the "slow trash" is already optimized to death - it just dies under the weight of insane complexity (e.g. features), browser support, and overly-generic (IMO) support libraries

> which requires a massive slow polyfill in all but latest Chrome and results in a shit experience for all others (if i recall a related thread correctly).

Tangential: I'm not sure which version of Polymer they're using - but 1.0+ removes the need for most of the polyfilling.