Alright so I got around to testing this out (on elementary OS 5.0 Juno). I found it utterly impractical to use for anything more than 1k ebooks.
The biggest problem is that on startup bookworm loads all book covers one after another.
It takes forever until every cover loaded for huge libraries.
At around 2.5k ebooks the interface becomes so laggy that it’s unusable. Scrolling is no longer possible.
I gave up testing this with 5k ebooks.
Another thing I dislike about this application is the inability to adjust the width of the columns in the list view. There are some books in my library with very long titles. Bookworm seems to use the longest title to set the width of the title column for all books.
The memory usage got to above 11% when idling after starting the App with 5k books.
But I don’t have direct access to my library the next few days.
Calibre can handle libraries much larger than mine as long as you remember to close the tag browser and such (takes waaay more time to start Calibre if tag browser is opened).
Just to be sure I just tried it via iOS Safari and signed up for a new account by entering a username and a password. Left the email field blank. Works like a charm.