One thing I've noticed is depending on how your code is being compiled/transpiled, having smaller defn's over them being nested in a let can lead to better performance since inlining becomes an easier problem.

This is particularly true in clojure, since it's being compiled to java byte-code, and the jvm has a few default settings for how deep it'll look into the call tree for inlining. See https://www.youtube.com/watch?v=0tUrbf6Uzu8 for a talk that mentions this.