> 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.

> 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. But it can't need any less reflection than "count" needs, and "count" is 100x faster. Surely whatever alength is doing in the non-type hinted case can't be... ideal? > 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! But where do all those clock cycles go? Doing runtime type inspection and then dispatching to one of 9 static functions based on that doesn't feel like it ought to be anywhere near that terrible (especially in a JIT'ed language).