You're using a good GB switch. My first thought at this point is to examine your cabling. Poor cabling can substantially impact throughput performance. Are you using Cat6 cables? Damaged or improperly shielded cables will hamper network performance. You may want to try using new Cat6 cables on the DNS-230 and BB.
During your testing, when you moved your devices from the switch to the router (or visa versa) were the same cables connected to the DNS-320 and BB or did you move the devices and use different cables? If so, you may want to try moving the devices along with the cables they are currently attached to and measure performance
