It's a bit more complicated than that. Loopback implies that the data went off the wire and was sent back. That puts some serious constraints on the process that deals with that data, how it is stored and how it is fed back into the system. You can't just compare that with naked pass-through latency (which is what a switch would do), because the signal is already on the way out before it has even fully been received.

Essentially a few ns after the header is received it will already be passed on to the output port after some signal conditioning, which happens almost without latency. A typical 'high speed' switch will have 800 ns round trip, so that 60 ns quoted here is actually quite impressive.