I now imported a total of 1.000.626 to create a stress test for the data base. I did not have that many images, so I re-imported the same images from a renamed path. Having that many database entries to test, I was able to to improve the database. So the upcoming new version will be even faster than the previous version.
After importing roughly a million images with the thumbnail size set to “large” the thumbnail storage grew to a total size of 125GB. The database is about 500MB. The “large” setting is intended to allow satisfying preview and browsing, also when the original image files are not available, so it stores about one megapixel and about 120KB per image.