Are we talking RAM here?

Yes. It isn't slow when loaded in RAM. But we don't want to keep it loaded in RAM, because paying for hosting with a lot of RAM is more expensive. And also because every time someone makes a post, that's more RAM we need, so the longer the site goes on, and the more people posting, it just keeps needing more and more RAM.

HDD space is much cheaper than RAM, so we want to keep all the data on the HDD. We can do that with SQL, which can quickly search and load specific data on the HDD as-needed. We can't do that with the current s-expression files.

Were/are there any alternatives to Racket that you see fit for something like Hubski?

Yes, but mk particularly likes Lisp, and I don't disagree. Common Lisp and Clojure would also be suitable; they're both comprehensive, popular, and well maintained. But since Arc is built on Racket, we can call Racket directly from the existing Arc, and thus move the site to Racket one function at a time. With any other language, we'd have to rewrite the site all at once, before we could use it.

As a software engineer, other languages I think would work well are Elixir, Go, Scala, and Nim.

Another possibility would be a drop-in Content Management System such as Wordpress or Plone; or a drop-in SQL REST API such as PostgREST, with SQL views and users, and the "site" entirely client-side HTML and Javascript.