HOLD it. 'dropping bandwidth...the connection itself is.. unused', do you mean dropping Frames Per Second? Bandwidth is NOT the same thing as Frames Per Second you see the movement on the screen and see it 'drop'. Bandwidth is was I described, the pipe that is for IN/OUT of a connection (Internet, LAN, etc.) of / from computers.
Now for FPS that is totally different. That is based on many thing, first off you "tested in an empty server", are you testing by sitting AT the 'server' and playing it from there? Or you have a SECOND PC connecting to the server for this test and seeing this results? If it is a second PC, how is both the Server and 2nd PC connected to the router? Are both Server AND 2nd PC in the same room or Server your house, 2nd PC at friends house?