When reinstalling, upgrading or moving settings, thumbnails may be missing in Shotwell. The problem can look like this:
Each time you import photos, Shotwell will generate thumbnails in two different sizes. If you loose them, you have the problem that they are regenerated on-demand, everytime you scroll by an image. That stinks.
You need to do
apt-get install sqlite3 imagemagick to fetch requirements.
Here's a script that ...