"Now Rainbow.js and Nu have come along and challenge that"
While Nu is drastically faster in certain areas, like the + function, in general it's slower than Arc 3.1 (by my estimates, about 5-10%). Unfortunately any Arc implementation built on Racket will have a hard time beating Arc 3.1's speed, simply because Arc 3.1 already pushes Racket pretty far. To get faster, I think you'd either need to find some serious oversight in Arc 3.1, or use Racket's modules.
Oh, sorry. :) Because of your your active work to make it fast, I figured it would challenge at least Arc 3.1, if not Rainbow, and I guess I got carried away. :-p
While you could probably trivially start from Arc 3.1 and streamline things here and there (like '+), you have a point about Nu adding other features.
That's right, and because the compiler is only a (small but important) part of Arc, I can only do so much. Arubic, however, can potentially be a lot faster than Arc 3.1, because it makes many more changes to the language. But that's a different topic.
Anyways, the only real reason Nu would be slower than Arc 3.1 is because it wraps every global Arc variable in a function. This is so useful that I consider it worth the 5-10% performance hit.
