> Honestly that's only an implementation dependent detail.
Not really, no. JavaScript is a ridiculously high-level target for another language, and this introduces a huge amount of complexity in any effort to target it efficiently.
You can pare down JavaScript, as Eich seems to be banking on, but what possible technical reason is there for this? If what you care about is compatibility with Apple/MS browsers, then generate JavaScript from your intermediate bitcode/bytecode, and let those browsers be slow and complicated and have worse development tooling.
Honestly that's only an implementation dependent detail. If there is a fast compiled Lisp like SBCL, a fast JavaScript can certainly be done.