TCP has a lot of rules nailed down in numerous RFCs - everything from how to handle sequence numbers, the 3-way handshake, congestion control, and much more. T

TCP has a lot of rules nailed down in numerous RFCs - everything from how to handle sequence numbers, the 3-way handshake, congestion control, and much more. That translates into a whole lot of code that needs to run, while unix sockets are not that much more than a kernel buffer and code to copy data back and forth in that buffer - which doesn't need a lot of code to make happen.