Loading a folder with many images takes ages which makes me assume that thumbnails are generated on the fly instead of using the earlier generated preview images.
Is there a way to test/view where the (thumbnail) images come from?
Afaik, you should find that information in ther Nextcloud log file. It is usually ben logged if a preview is generated.
You can copy e.g. 100 pictures in a new folder and compare first and second access.
@devnull what difference would you expect? If it is generated on rhe fly, that generated preview is not stored either, is it? Thus it will be regenerated on the fly each time I access?
Normally Nextcloud generates at first access previews:
https://docs.nextcloud.com/server/19/admin_manual/configuration_server/config_sample_php_parameters.html#previews
Also you can use a preview genenerator:
https://apps.nextcloud.com/apps/previewgenerator
You find the preview images nextcloud uses in
data/appdata_xxxx/preview
(appdata_xxxx … xxxx is another name)
You also can for testing empty the folder “preview” and then nextcloud must make new previews. After that there must be new preview picutres. If not there is an error.