I have an 8 year old Kobo that had been struggling with several books recently, but the Secret Project books were the absolute worst. Some of the pages would take 30-60 second to load and then only display a small corner of the artwork.
I have been using Calibre on my PC to handle my ebook library for years. The software has the ability to add plugins that provide more functionality. I recently found the Kobo Touch Extended plugin which automatically converts ebooks to a better format for Kobo when you load them onto the reader. All the art is scaled down to for the screen size, along with some other conversions.
It has made a night and day difference for me. Pages load almost instantly, even when they are just artwork. I honestly want to read Tress again now that I can see the art fully.