In short: it's slow because there isn't just one kind of Java array, and if you don't say which one you mean, Clojure has to do reflection to figure it out.

Inside the JVM, there are several different types of arrays: bytes, floats, objects, etc. Clojure.lang.RT includes methods for getting the length of all of these types--for instance, here's the method for the length of arrays of longs:

https://github.com/clojure/clojure/blob/master/src/jvm/cloju...

The clojure.core/alength function calls clojure.lang.RT/alength, but that callsite is polymorphic: the Clojure compiler doesn't know which of the RT functions to emit a call for, because it needs the type signature. If the type is unavailable at compile time, the Clojure compiler emits reflective code which inspects the type of the reference, determines which specific clojure.lang.RT/alength implementation to dispatch to, then executes that call dynamically. That's the slow part!

https://github.com/clojure/clojure/blob/clojure-1.10.1/src/c...

Just like the article says, if you include a type hint, the compiler can emit an invokestatic call directly to, say, clojure.lang.RT.alength(long[]), and skip all the reflection.