I love that this Lisp version from a comment at lobste.rs (https://lobste.rs/s/xz5l8t/one_program_written_python_go_rus...) runs in less than half the time of Rust, and is more readable to boot:

    (declaim (ftype (function (string string) double-float) img-diff))
    
    (defun img-diff (first-file-name second-file-name)
      (declare (optimize (speed 3) (safety 0) (debug 0) (space 0)))
      (let ((im1 (png:decode-file first-file-name))
            (im2 (png:decode-file second-file-name)))
        (declare ((SIMPLE-ARRAY (UNSIGNED-BYTE 8) (* * *)) im1 im2))
        (/ (loop
            for i fixnum below (array-total-size im1)
            summing (abs (- (row-major-aref im1 i)
                            (row-major-aref im2 i))) fixnum)
           (* (ash 1 (png:image-bit-depth im1)) (array-total-size im1))
           1.0))) ; Convert rational to float
    
    (img-diff "file1.png" "file2.png")