Thank you for the extensive information on alternative viewers, BR!
Anyone know if there are plans in the near future to make improvements to the Viewer? The super slow issues still exist in the Calibre 5.42 viewer with some books, primarily older ones but not always. The problems range from slow initial loading to extreme lag (more than a couple of minutes) moving past 0% or 1%. It largely tends to be older books but not always.
I've tried re-downloading and re-adding some of my books from Amazon to Calibre, which hasn't really made a difference so far. I also tried converting some of the older MOBI books to AZW3, which helped a small handful but not nearly as many as I hoped for. I just wish there was some way to address a large quantity of books. Yes, it's unrealistic. Too many books from too many sites over many, many years just aren't going to magically optimize themselves, darn it.