It's known that after Firmware 3.2 Kobos suffered a major performance degrade on certain PDFs.
Here's a link to the thread I started on it way back in the day, complete with amateurish YouTube videos demonstrating a comparison between 3.2 and 3.8. The most recent firmware that we specifically looked for the problem was 3.17, but I can tell from my fully-updated H2O that nothing has changed.
Here's a link to my post where I finally found a freely-available PDF that demonstrates the problem perfectly.