Are you guys seeing this on the dns-323?
Neither the DNS-323 or the DNS-321 operate at full theoretical gigabit network speeds.
www.smallnetbuilder.com has a NAS performance charts that rate the D-Link DNS series NAS at various
network settings (100/1000), jumbo frame settings 4K (on/off), RAID configuration (JBOD/RAID0/RAID1),
and read and write performance. You should judge your performance against the www.smallnetbuilder.com results ~15MBytes/sec, not the theoretical gigabit network speeds (1000bits/sec).
If you want a faster NAS you need find one with more RAM. The DNS (321 and 323) series have 64MB of RAM this contributes heavily towards limiting your network transfer performance. If you want a very fast dual drive NAS look at the Synology DS209+ it has 512MB RAM with ~40Mbytes/sec transfer rate, and costs over 2.5 times the price of the DNS-323.