The syntax of your language is quite nice. I’d maybe change (for …) to (each …), (fun …) to (fn …), and (let …) to (def …) or (set …) depending on implementation details of variable assignment, but those are just aesthetic preferences :)

I love '{thing}' for string interpolation.

If you haven’t already, check out clojure, janet-lang, io-lang, and a library like lodash/fp for more syntax and naming inspiration.

Keep building!

> reducing its execution time by 7.03 times or 703%.

Shouldn't the latter be by 86 % ?

Somewhat OT, but I cannot infer the semantics of let from the examples given. Among other things it seems to have indefinite scope.