apeschar
Of course, none of those factors can be influenced by the site itself, and all of those factors influence all sites at the same time. We're focusing on things we can improve.
Although it's a simplification, I believe it's broadly correct to categorize things this way. The main reason to highlight this is because oftentimes people (clients) will focus too much on the "heavy page" part, because that is what measurement tools like Google PageSpeed Insights mostly tell you to improve.
The one "logical" roundtrip (an additional request) would indeed count for many if you consider DNS, TCP and TLS handshakes.
Even more of a reason to get rid of them.
This plugin also defers all scripts and tries to simulate all the events that occur during a normal pageload so that the scripts still run as they normally do.
For example, you use Google Fonts. For that you need to load CSS. So that's three roundtrips for rendering your page: 1) your page, 2) the Google Fonts CSS, 3) the font files.
We've build a WordPress plugin called PhastPress (fast press) that helps you reduce the request count and those roundtrips without building your own theme. https://wordpress.org/plugins/phastpress/
Among other things, it inlines the critical CSS needed for rendering the page, defers all script loads, and optimizes images. Cuts page size by ½ on the stock theme, and the amount of additional roundtrips to 0.