You are testing the same files both ways?
Are you accessing the NAS through SMB or a DLNA server? If the latter, do you know what kind?
Is your router a 10/100/1000 router or just a 10/100. The reason I ask is you can buy a compatible unmanaged switch or hub to expand the amount of ports available to you. You just need to buy one that matches closely to the speed of the router. Either a 10/100 or the gigabit one.