Comment 3 for bug 1846852

Yes, webengine is a new thing, see
https://calibre-ebook.com/new-in/thirteen

There is already viewer functionality to have a single instance,
so then every time you trigger the viewer the book will open in the
existing viewer. I just haven't got around to exposing that option in
the viewer preferences, as yet.

 As for the actual startup delay, it definitely does not sound normal, I will
 investigate after the craziness from 4.0 dies down.