@davidfor:
Here's the link. To be fair, it's a large file and even the preview is 15MB in size and full of ToC entries, though I have examples of (sideloaded and converted) kepubs that are under 2MBs which error out completely, and for which even a restart doesn't fix the problem. While I believe that your assessment is actually correct, what struck me initially is that the preview experience was not different from the one I had when converting that same Delphi epub file to kepub.
I've seen (physically) much larger kepubs be able to open instantly, while others, notably smaller, I simply couldn't get to work. While I'm aware that epubs and the code aspect of the problem is an often incomprehensible maze, and that it's impossible for any solution to be perfect, the fact that Kobo itself has that kind of file made me feel a bit more optimistic that it is, in fact, a bug, and not some natural and expectable consequence of potential conversion pitfalls.
Currently, I only convert footnote-laden and highlight-worthy files and push through the reboots because of the vastly superior experience on the kepubs. I wouldn't even be insisting on this issue so much, were it not for a gnawing sense that so many errors and restarting are ultimately bad for the device.