I’m not who you are responding to, but optimizing recursions (e.g. tail-call elimination), loop fusion, etc. Functional languages (at least haskell) does plenty of optimizations not generally possible in imperative codes.
tail call elimination is possible by converting the tail-recursive function to a loop. in fact Go could support it, but the team doesn't want to do that.
I agree that everything Haskell does would be hard, but everything e.g. Clojure does should be possible
