A Windows 7 PC had no problems connecting to a Synology DS209. We recently switched to a D-Link DNS-321. While it connects/maps the drive without a problem, Office files (and I think others, but Office is my main program) take an absurd amount of time to open, save, etc.—fifteen to twenty seconds.
Nothing was done differently from connecting to the Synology. The Mac in the office has no problems connecting to it—files load as quickly as you’d expect. There is a lot of equipment on the system, but all switches are Gigabit-capable. Also, the Synology and the D-Link follow the same network paths (i.e., they’re connected to different ports on the same switch). This issue followed a move from one end of the house to the other, so changing the middle networking components had no effect.
Other than moving the equipment around to check the equipment/lines and testing the setup with the Mac, I don’t know what else to do diagnosis-wise. Given the result of those steps, I’m inclined to think it’s a Windows’ settings issue, but have no idea where to look.