How many round trips does a full TLS 1.3 handshake add before the client can send its first HTTP request, compared with TLS 1.2?

A full TLS 1.3 handshake takes one round trip, and TLS 1.2 needed two. TLS 1.3 is faster because the client sends its key share in the very first message.

TLS 1.3 also has an optional 0-RTT mode for resumed sessions. In this mode, the client sends data with its first message. But an attacker can replay this early data. So it should only carry idempotent requests, meaning requests that are safe to run twice. That is why "TLS 1.3: 0 round trips always" is wrong.