I gave one of my backups another try: duplicati is now running on my windows laptop (ssd / core i7 / 8gb ram)
This time everything went very fast and it already could recover half the storage (40GB recovered from 80).
so it looks like that there is a problem with duplicati on arm devices. if anybody wants to troubleshoot it feel free to contact me 
This restore is also running from local drives. All restore tests from the internet (sftp and jottacloud) are not usable due to missing parallel downloads. The download would take days…