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.

Yes because we just add the things we need, at whatever layer it makes the most sense to add them.

This type of application stack that includes the language you're writing in and even, when convenient, its implementation, is really satisfying to work with. There is much less need for workarounds, arbitrary choices, and various indirections (e.g. what used to be called dependency injection, for example). All the plumbing is an order of magnitude simpler and it allows us to keep the codebase much smaller than it would otherwise be. I also spend basically zero time bitching about having to deal with software dependencies, making me realize how much of my former life as a programnmer was taken up with just that.

I think of this as sort of the unikernel form of application dev and of course it's a fine fit for a Lisp, since "write the language you want to write your program in as you write your program in it" is the natural style there. The tradeoff is that there's a lot of vertical coupling between the layers. If you want to factor out one layer for general consumption, e.g. to open source the language implementation so other people can build cool things with it, there's a fair bit of work to do.

Also, since the language implementation exists to run a specific application, we just don't bother supporting we don't need for HN. That too comes back to bite you when you want to open-source it!

HN has had 15+ years of work since the original news application was open-sourced; that's a lot of things-we-added at-some-point. Most of those are at the application level but some ended up in Arc and some in the Arc implementation it was convenient to put them there. This is especially handy when you have limited time to work on the code.