My 2012-era MacBook Air is connected to a 2012-era iMac via a Thunderbolt cable. The iMac contains a Thunderbolt-Ethernet bridge which in theory should allow the MBA to (1) participate in extremely fast file transfers with the iMac and (2) access the Internet and LAN without the use of additional ethernet cabling or WiFi.
Both operations work as expected, but with a catch: Sending TCP/IP traffic from the MBA to any computers on the LAN other than the iMac, or to any computers on the Internet, is incredibly slow. Receiving traffic works just fine. Sending a file from a server on the LAN to the MBA, for example, is very fast, and downstream speed tests initiated on the MBA via SpeedTest.net max out my Internet connection at 60Mbps. But all traffic originating from the MBA in the upstream direction is ridiculously slow; Traffic from the MBA to machines on the LAN or the Internet moves at around .08Mbps. A test via SpeedTest.net confirms. (Traffic between the MBA and iMac is very fast in both directions.)