I wonder if its related to this ==>>
https://www.mobileread.com/forums/sho...d.php?t=193686.
The cause of the OPs problem starts to reveal itself at post 33. The solution is at posts 50/51. And if it solves your problem send some karma to PeterT.
Otherwise, as a quick test you could try creating another library with no customisations, copy say 20 books from existing library but DON'T create missing custom columns - restart calibre
If calibre is still slow to start then the problem is probably NOT related to the 30K+ library. What happens if you start calibre in Windows safe mode?
BR