When you loop back Ethernet links in the same computer, you need to take care with the configuration, because normally the operating system will not route the Ethernet packets through the external wires but will process them like being for localhost, so you will see a very large speed without any relationship with the Ethernet speed.
How to force the packets through the external wires depends on the operating system. On Linux you must use namespaces and assign the two Ethernet interfaces that are looped on each other to two distinct namespaces, then set appropriate routes.
How to force the packets through the external wires depends on the operating system. On Linux you must use namespaces and assign the two Ethernet interfaces that are looped on each other to two distinct namespaces, then set appropriate routes.