It still uses Arc. The original implementation was written in Racket, then rewritten in Common Lisp and run on SBCL.
I’m curious: was Arc running Racket BC or CS? I understand it got a big performance boost after switching to Chez Scheme.
dang: It was running BC. I had high hopes for switching to CS because I'd heard the same thing you had, but when I tried it, HN slowed to a crawl. This stuff is so unpredictable.
I can't remember the details, but I recall that Arc was pinned to an older version of Racket and didn't benefit from improvements to BC and then later to CS.
But ... be aware that the Arc interpreter is much more dynamic than Racket, so in general normal Racket programs are more efficient than Arc programs.