Round Trip Time

a very good question, to which I have no definitive answer as we’re talking network-level here which is not my speciality. my understanding;
a HTTP-request uses TCP which in turn uses IP. I guess that a extra round trip is needed when the size of the full HTML (or of the HTML HEAD + the above the fold BODY) is larger then the size of the segments (TCP) and/ or packets (IP).

if you have time, have a look at https://en.wikipedia.org/wiki/Transmission_Control_Protocol and https://en.wikipedia.org/wiki/Internet_Protocol and don’t hesitate to correct my (mis-)undertanding of the subject 😉

frank