Serviceworker for thumbnail-picts

[tackin]

It would be nice if a service-worker on a slow Raspberry Pi could do some work at "night". I uploaded a big set of JPEGs to a space. If someone opens the folder with the picts for the first time, opencloud starts building thumbnails, which lasts some time and lead to a high load on that old raspi and it gets very "unresponsive".
I would like to have a serviceworker in a single thread that looks inside all folders an builds the thumbnails if not existing in the night when there is no other request to serve for the Pi.
Sorry if that is already existing as a variable to switch on. I spend some time for a look into the docs and could not find something about it.