From a bit of debug printing and experiments, it does appear that unlike let, loop replaces the type of primitive int with long, and of float with double: https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/Compiler.java#L6385-L6397

Again, I still do not know whether that is causing this performance oddity, but it is different than what the docs say here: https://clojure.org/reference/java_interop#primitives

https://clojure.atlassian.net/browse/CLJ-1905