TCP Handshake Time

TCP handshake time is how long it takes the browser and server to establish a reliable connection; on HTTPS, setting up the secure (TLS) connection is added to it.

This is the second link of the chain, after the DNS lookup and before the request reaches the server, and like DNS it depends mostly on the network and the server's geographic distance, not your code. On today's sites, all HTTPS, this stage is really two jobs: the TCP handshake and then the TLS handshake for encryption; the farther the server is from the user, the more expensive these round trips get, and this is where a CDN helps by moving the connection point closer. Like DNS, on a reused connection this stage returns zero, so the denominator is only fresh connections. The measurement nuance is that separating the exact TLS share from TCP needs data not every tool collects; often the two are reported together as a “secure connection time.”