Technically, a full handshake adds 2 RTTs, and it's only one RTT on a successful resumption. In practice, I think that most modern browsers currently use something called False Start, where they cheat and send some data early, so even the full handshake takes only one RTT. False Start works only with a well-configured server.

In TLS 1.3, the full handshake is one RTT without any tricks; resumption also. There is also a 0-RTT resumption, but it sacrifices security for speed. Lots of people are unhappy about it.