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.

Just to confirm, I opened ``Transmetropolitan - I Hate It Here (2000) (c2c).pdf'' for a quick read.
Start up took about 5 or 6 seconds, but after that the pages all loaded instantly even though I scrolled down pretty fast.

>>31,32
Can you post a link to that file? I'm almost willing to bet it's not JPEG2000 but regular JPEG or even PNG, both of which are very very fast.

For comparison, this is regular JPEG:

https://archive.org/download/TheVoynichManuscript/Voynich_Manuscript.pdf

This is JPEG2000+JBIG2:

https://archive.org/download/hydraulicelevato00baxtrich/hydraulicelevato00baxtrich.pdf