For what it may be worth, my calibre install used to shut down too fast to notice any delays. I've used the
Backup configuration folder(s) plugin installed for close to a year now and it does slow my shutdown so it now takes ~12 seconds to shut down.
Please note that this delay is not a "
MAJOR BUG" in calibre but rather the result of having to create and save a .zip archive of your current configuration folder. In my case, the majority of the .zip file is occupied by a backup of my current plugins. About 99.4MB of data is stored in the 81MB .zip file.
One acquaintance who has a older system and about 315MB to backup measures his shutdown delay around 50 seconds but he likes the secure feeling from having his calibre configuration automatically backed up.
As @BetterRed said, if you are storing other files in the calibre configuration directory or have issues with the cache not being cleared, shutdown times can skyrocket.