I had fun with this one. Tried all sort of things. strace helped in that it showed lots of file activity when it was hanging. I have lots of books. Many live in zip files with many books per zip. I first tried telling it not to scan for books - it still hung. Then I removed the paths (library tab) it had to scan for books. Now it works but is very slow when entering the a 'books' dir or when responding to the selection of a book. Looks like it always tries to scan if a path exists.
The network support is nice. Any chance it could be configured to understand a calibre web service? It would need to know an addressort and user/password. Here this would be very handy as I try to keep my books indexed with calibre.