I agree. For Volume_1 to Volume_2 transfers (i.e. with the PC as an intermediary in the copy) your throughput is within an acceptable range. If you are uploading from your PC to the DNS-323, I would expect your throughput to top-out at around 7-12 MB/s, which will depend on the network hardware, cabling, and settings, etc. For downloads from your DNS-323 to your PC on a gigabit network, I would expect a maximum throughput in the low 30 MB/s range, again depending on network hardware, cabling, and settings.
There is a known issue with Windows XP (at least on the DNS-343), where throughput chronically drops to the 2-5.5 MB/s range. For some users, a reboot corrects the issue, for others, this speed is persistent even after reboot. I've experienced this issue myself until migrating to Windows 7.