And likely that the QNap NAS is choking on all of the work it is doing over iSCSI. The QNap is probably running a RAID5 or RAID6. If the database and the storage are both on that volume then it is choking on disk contention.
When we evaluated UrBackup it was on a bare metal machine that had a RAID5 split in to two volumes. OS on part of it and a storage volume on the rest of it. Performance was pretty horrible. Now that we have moved to an SSD RAID1 for OS (and therefore database) and a RAID6 for storage, performance is massively improved. If they are both on the same volume, it cannot do database operations and storage operations at the same time. You run in to serious disk contention.