Scan for new items takes ages after syncing collection btw. two computers

[Jim Gomi]

I would strongly advise against including the digikam4.db file in the
synchronization when syncing between two different machines.
I ended up with a corrupted database file that way, and it was a lot of
effort to un-corrupt it
https://mail.kde.org/pipermail/digikam-users/2017-January/023348.html

It's much safer to configure digikam to write all tags etc to the image
files, and then just sync the image files. 
It's true that after syncing you have to wait while the database is
rebuilt, but at least you know that you'll end up with a clean correct
database.