Arc was re-written to Common Lisp (SBCL) around last year, for performance reasons.
> [Clarc] is much faster and also will easily let HN run on multiple cores. It’s been in the works for years, mainly because I rarely find time to work on it, but it’s all pretty close to done.
> there’s now an Arc-to-JS called Lilt, and an Arc-to-Common Lisp called Clarc. […]
> open-sourcing the Arc implementation (i.e. Clarc) would be much easier [than the HN site]. [Because…]
https://en.wikipedia.org/wiki/Arc_(programming_language)