Best was is to generate previews via cron, it will only than download generated one. Check this out: How to know if Preview Generator is working? - #6 by gas85
Also here is an old tread with config How to clean up nextcloud-data/appdata_* and /updater-* - #7 by gas85 and explanation to understand previews: How to clean up nextcloud-data/appdata_* and /updater-* - #9 by j0hn