The handshake, and what it costs

A TCP connection opens with three segments. The client sends a SYN carrying its initial sequence number; the server answers SYN-ACK with its own and an acknowledgement of the client's; the client sends a bare ACK. Only then can data move.

That is one full round trip, spent before the request exists. On a 20 ms path it is 20 ms; from London to Sydney it is closer to 250. A client that opens a fresh connection per request pays it every single time.

The SYN and the FIN each consume one byte of sequence space while carrying no data. That is the source of nearly every off-by-one in a TCP implementation, and it is why the acknowledgement in the SYN-ACK is 1 rather than 0.