[DVdm]

I think I found the cause of the extremely slow opening of epub-books with the Calibre viewer, an annoyance that has been there for ages.
Some books indeed take over 5 minutes to open, and some I have to interrupt and decide that they probably will never manage to open. I created a test book with one sentence on one page and tried opening it. It took 3 minutes to open.
On the other hand, when I open a book by going to the epub-file folder and double clicking it, it opens within seconds.

With the task manager I noticed that, when opening outside of Calibre, there's a process called "The calibre e-book viewer", whereas when I open an epub from within Calibre, I get a process called "calibre worker process".

So, I tried this:
Go to 'Preferences', 'Behaviour', 'Use internal viewer for', UNcheck (!) the entry for EPUB, which was checked on my system (Calibre portable 6.12 on WIN10 Pro).

When I do that, all my e-books open immediately with a process called "The calibre e-book viewer" and that horrible delay is no longer there.
PHEW, what an infinite relief!