It obviously relates to Windows 7, rather than the DNS-323 - have you looked around the various Win7 fora?
I don't have a good answer to your question, and in fact am puzzled by other Win7 related performance issues when accessing files stored across a network - I've been led to believe that Win7 handles network attached storage (and I used that term to encompass any & all storage not locally attached) differently, and attempts to use some form of encryption, which may not be supported (more frequently not), which is apparently one cause of reduced throughput - this of course does not explain the degradation that you mention.