When I start a mc server (exe or jar) on my computer and connect with the client on my computer offline (no internet connection), the connection with localhost or 127.0.0.1 is very bad (only one bar 15000ms and above). If I'm online and connect to the server on my computer over localhost, the connection is good (all bars 65ms). This is since the lan connection is in the game. The connection between client and server on the same computer should be offline as fast as online.