This reads a bit like "why you shouldn't use clojure" to me. I'd expect to leave some performance on the table when writing idiomatic code in a dynamic language with immutable datatypes, but a function that's already specialized to arrays, as the name indicates, taking 3 orders of magnitude too much time? 1-10 microseconds to say "get me the length of this java array, and BTW this is a java array"? What on earth is going on there?

Edit: So I just ran the author's (alength (long-array 5)) (with the indirection to foil type inference) and got 10µs mean execution time in criterium. By contrast here's some roughly equivalent python:

   In [5]: a = array.array('l', [5])                                                                                                                                                                         
   In [6]: timeit len(a)                                                                                                                                                                                     
   32.9 ns ± 0.105 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

More or less exactly the same with a plain python list. So clojure's type specialized function manages to be 300 times slower than the generic version in freaking *python*. In fairness "count" is better if still terrible (4x slower than python).