This paper is an interesting read, and explains how Koka is fast: <a href="https://www.microsoft.com/en-us/research/uploads/prod/2023/05/fbip.pdf" rel="nofollow noreferrer">https://www.microsoft.com/en-us/research/uploads/prod/2023/0...</a>. In short: they use a clever algorithm to transform functional code into imperative code that reuses memory where it can.
This post by the Koka-author is an update about what's currently being worked on: <a href="https://github.com/koka-lang/koka/discussions/339">https://github.com/koka-lang/koka/discussions/339</a>