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.

https://archive.org/download/JBlakeboroughAndSonsLtdCCA15302/J%20Blakeborough%20and%20Sons%20Ltd%20-%20CCA15302_text.pdf

Time how long it takes to render each page.

JPEG2000 has impressive compression ratio as well as CPU usage.