Nothing magical. Just a connector. My Nexus 5X has a USB-C connector on its USB 2.0 port.. 480mb/sec max, same speeds as one would expect from any other type of USB 2.0 port.

Ditto when a USB 3.0 system has a USB-C connector: still "only" 5gb/sec max.

Now.. the new notebooks that sport USB-C ports, generally implement USB 3.1, which is faster: 10gb/sec. But they could do that with a USB Type-A port just as well, though clunkier.

Cheers