When you visit a website, your browser connects to a server via TCP. With TCP, the first roundtrip can be up to 14 KB large.
In other words, the first 14 KB sent to the client will always be the quickest to render in the browser. The rest of the response is streamed afterwards.
This website’s homepage is about 9.7 KB at the time of writing. Articles are roughly 4-10 KB, depending on their lengths. All CSS is inlined, so besides fonts and images everything is loaded withing the first roundtrip, making page loads fast and snappy.
Not all sites can be contained within 14 KB — most probably can’t. But keep the number in mind, and try to optimize the first 14 KB instead.