I cannot replicate the issue in my
AuraHD with 3.8.0.


I mean I've got some big pdf files and, in a 90Mb one (mainly text with embedded pictures) and in another 20Mb big one (scanned pages, so just pictures), the page turn or loading book times are about 2-3 s at most. (I didn't use a stopwatch, but I can assure you it doesn't take the huge times you are posting...)
(As I don't usually read pdfs, I don't really remember if 3.8.0 is slower or faster than 3.2.0... I just remember that, at last, landscape navigation is fixed...)
Something really weird here!!!
Just in case,
please check through Kobo Utilities plugin for Calibre
that your database is not corrupted.
EDITED ----
Nevertheless, testing this issue I think I've found another bug about pdfs with pages of different sizes, but I'll perform more tests.
EDITED (2) ----
I've opened
a new thread for this issue in different page sized pdfs.