I too was surprised that OP's imgdiff is "3X faster than the fastest in the world pixel-by-pixel image difference tool". I would have expected imagemagick etc to be faster.

Here's some Python code (using OpenCV & numpy) that is roughly equivalent to OP's imgdiff. It is single-threaded but it'll be vectorised and it's 3x faster on my 5-year-old 2-core laptop (even counting the Python overhead):

  import sys, cv2, numpy
  
  f1 = cv2.imread(sys.argv[1])
  f1 = cv2.cvtColor(f1, cv2.COLOR_BGR2GRAY)
  f2 = cv2.imread(sys.argv[2])
  f2 = cv2.cvtColor(f2, cv2.COLOR_BGR2GRAY)
  threshold = 0.1
  
  absdiff = cv2.absdiff(f1, f2)
  _, thresholded = cv2.threshold(
      absdiff, int(threshold * 255),
      255, cv2.THRESH_BINARY)
  
  cv2.imwrite("output.png", thresholded)
  print("Different pixels: %s" % numpy.count_nonzero(thresholded))

Benchmark:

  $ /usr/bin/time python test.py water-4k.png water-4k-2.png
  Different pixels: 89153
  1.39user 0.34system 0:01.33elapsed 129%CPU

  $ /usr/bin/time ./imgdiff water-4k.png water-4k-2.png output.png
  Failure! Images are different.
  Different pixels: 89142
  Command exited with non-zero status 1
  8.85user 0.10system 0:04.02elapsed 222%CPU

(Edit: The Python code is a simplified version extracted from https://github.com/stb-tester/stb-tester -- we use it for regression testing of GUI screenshots.)