Are Thumbnails ever invalidated?

I have a site with a couple of galleries on it.
I let Kirby generate quite a few thumbnails and that takes a while. Sometimes a page needs very long to load even though the thumbnails had initially been creating a couple of days before.

Are thumbnails ever invalidated or automatically deleted? If not, what could be the source of these performance issues?

No, thumbnails are not deleted or invalidated automatically by Kirby.

The filename of a thumbnail is based on the file path and the settings (width, height etc.). So the thumbs are not generated again if the name and code don’t change.

But there’s one catch: If the modification date of the original image is newer than the one of the thumb, the thumb filename stays the same, but the file gets overridden by a new thumb. Maybe this can be the issue.