KFileItemModelRolesUpdater: Generate thumbnails for visible files first
Folder thumbnails are not cached as we cannot know whether something inside a folder changed. Moreover, they are also a collage of several files and might even traverse into subdirectories. This takes time.
Another shot at https://phabricator.kde.org/D14770 but only affecting the visible area to address the concern of jeopardizing the case of a folder with primarily subdirectories.
Edited by Kai Uwe Broulik