The improved Swift code shown in the original post takes 0.5s to compile with optimizations on my system. It runs in 4-5us per inner loop.

The following Common Lisp code:

  (defun bench-array ()
    (declare (optimize (speed 3) (debug 0) (safety 1)))
    (let ((arr (make-array 3000)))
      (declare (dynamic-extent arr)) ; stack allocate
      (loop
        :with sum fixnum = 0
        :for i fixnum :from 0 :repeat 15 :do
          (time
           (setf sum
                 (loop :for k fixnum :from 0 :repeat 3000
                       :do
                          (setf (aref arr k) i)
                       :finally (return (loop :for l fixnum :across arr
                                              :sum l fixnum)))))
          (print sum))))

compiles in 0.01s and runs in 5us per inner loop on my system (SBCL).

It has array bounds checking enabled (safety 1 declaration). If I remove it (safety 0), runtime improves to 2-3us per inner loop.

I'll take Common Lisp over Swift any day of the week :-]