Ever wonder why software is so slow...?

Because it's all written by retards.

https://github.com/ArtifexSoftware/jbig2dec/blob/master/jbig2_image.c

Scroll down to jbig2_image_compose_unopt()... and then facepalm. It doesn't take a genius to realise that twiddling an image pixel-by-pixel is going to be dead slow.

How do you twiddle an image pixel by pixel without twiddling an image pixel by pixel?
B = op(A, B) = [op(A_0,0,B_0,0) .. op(A_w-1,h-1,B_w-1,h-1)]
How do you bypass a definition?