Thanks for your replies! To be clear, the backup speed is totally adequate for my purpose: I plan to use this to do monthly backups of the DNS-320L to store away from home. While the initial backup ran ~24 hours, subsequent increments should involve less data transfer and thus should well be able to run overnight. I actually prefer that to a network based solution that would force me to leave some computer running while the backup is transferring.
So, I was not actually looking for a way to speed things up but just wanted to know whether the speed I observed was as expected or whether it was a sign of a potential problem. I take your replies as a hint towards the former.
@redpiyaya: As stated, the backup speed was roughly 8.5 MB/s - not fast, but definitely above 2MB/s.