I'm experiencing this issue too! I've used many systems: official Apple Time Capsule, QNAP NAS systems, and this is by far the slowest I've come across, I mean really slow. For example transfer rates ranging from 1-5MB/sec it's takes forever to backup a 1TB SSD. I'm backing up two Macbook Pros, both have their individual disks in unRaid and have the AFP share set up as per this post:
Is there something I've missed or configured incorrectly?