I have done things on my site like inlining CSS and minimizing requests because it feels fast, without knowing exactly why. I didn't know that a single request can actually have multiple round trips!
My WordPress homepage is 2.4kb gzipped and just a single request :) so it is fast but its, uhh, a little boring and only 158 words.
If you load something from a third-party origin (e.g. 23789dz89asd789s.cloudfront.something), then it can actually be a lot worse than that. DNS needs resolving, which can take quite a while, depending on if and where things are cached. Then you get a full TCP handshake (since TFO doesn't work for the first connection), a full TLS handshake and then you get to roundtrip your request(s).