[SOLVED] Backup suddenly 6 times slower

Hallo,
I have a problem with the Backups of my machines. It usually took around 2:20 hours to backup the machines. Now from August it suddenly takes almost 12 hours.
Does anyone has an idea what could be the root cause?

I have 2 Mirror ZFS Pools:

the first nvme0n1 and sda, this pool contains the VMs and CTs (Internal)
the second one sdb and sdc, this pool contains the backups. (External USB3 Harddisks)

Thanks in advance
Gerd