Migrating huge installation (may delete previews?)

Nextcloud version: 13.0.1
Operating system and version: Ubuntu 16.04

I want to migrate nextcloud to a new hardware. I have created many previews with the preview-generator. The copying of the files takes some time, but I can manage it. But the dump and restore (as described in https://docs.nextcloud.com/server/13/admin_manual/maintenance/backup.html) takes too much time. The database dump is 336MB. The import on the new server is now running for 30 hours and still not ready (I started it with -v, so I can see easily that it is still working). Is it possible to delete all previews in my old installation? Or is there another way to speed up?

Thanks, Stefan