[kovidgoyal]

Move it off onedrive and you will be fine. Be as suspicious as you like, not going to change the fact that onedrive and calibre DO NOT get along. Windows has mandatory file locking, onedrive is scanning files inside caltrash which prevents calibre from deleting those files. calibre waits some time and retries each file delete to workaround this, hence the slow startup. Again, MOVE YOUR LIBRARY OFF onedrive.