Time to first byte
Time to First Byte (TTFB) refers to the time between the browser requesting a page and when it receives the first byte of information from the server. This time includes DNS lookup and establishing the connection using a TCP handshake and SSL handshake if the request is made over HTTPS.
TTFB is the time it takes between the start of the request and the start of the response, in milliseconds:
TTFB = responseStart - navigationStart