In a nutshell
When you load a URL the browser walks a waterfall of phases. First it resolves the name to an IP address (DNS), opens a TCP connection with a three-way handshake, and on https secures it with a TLS handshake. Then it sends the HTTP request and waits for the server to respond — the gap until the first byte arrives is the TTFB. The HTML then downloads and is parsed and rendered into pixels. Because each phase waits on the one before it, every round trip and every slow step pushes the first paint later, which is why shrinking this waterfall is what makes a page feel fast.
