I made quick Clojure version [1] it's really slow, and probably terrible, but I wonder if someone more knowledgeable in Clojure could look at it and optimize it the way it happened with the Haskell version
4.5 -> 2.7 seconds on my machine when using 'quot' instead of '/'.
2.4 seconds when hinting ^long in argument of collatz-length.
1.6 when also hinting ^long in collatz-next. Interestingly, when I hint only in collatz-next it takes 1.5.
I was also unsure if loop/recur was a good loop paradigm for this kind of problem (maybe for with variable would be better?) but I'd say in 1/3 speed of C it's pretty good for Clojure :)