I don't think the question is off topic, but I do think it is too broad.
In addition to that in my experience the load times the Q refers to, are reader dependent, difficult to measure on devices, and there might be multiple optimal configurations for an EPUB depending on hardware of the device (e.g. answer being CPU and memory speed dependent).
So the question could be better, and that is why I voted to close, in the mean time aware that not many. I didn't downvote the question, because I think it is a useful question, just not something very good as stated for this Q&A format.
On a starting site like this, voting to close might not be a good idea. We clearly can use some more questions. On the other hand broad questions threaten future 'sub-questions' from being closed as duplicate. That although such sub-questions could be better match someones visiting this site, and for sure would have a more specific answer.
An example sub-question could be
I am going to publish via X, primarily targeting device Y with EPUB files. My book consists of one page chapters 1.5 to 2K in size. Does putting each chapter in a separate file influence loading times in a positive or negative way, or is especially transition from file to file unnoticeable anyway.
I would probably vote to keep this open if it showed up in the close queue as duplicate of How can I optimize an EPUB file (closed or not), as I rather not have someone else with this 'problem' extract specific information from a lengthy answer to (IMHO) broad question.