I'd have to agree that it's probably something messed up in the PDF you created. I took screenshots of a nicely illustrated purchased children's book that I also wanted available on my ereader (one of the purchased books I posted about on
Page 10), the size is 86.5MB and it opens perfectly on my Kobo readers with 3.17.0 and I don't have the sickel patch installed. This PDF is all images, I can zoom in on every page, everything works as it should and never once times out (in fact it's pretty responsive, only taking 3 seconds to open up, page turns very fast as well).
And there's a larger purchased PDF heavy with images but mixed with text, 94.2MB in size, that also opens perfectly just as quickly, zero issues reading through the book.
So apparently it's not the size that's an issue, it must have something to do with how you created the PDF, maybe your images you started with, but something must be off. You can't blame Kobo, or any other ereader manufacturer, for that.