For the browser to be able to have enough HTML to render everything above-the-fold with only a single network roundtrip.
Specifically, by making the following less than 14 KB in total: HTTP headers, HTML <head>, basic skin layout (as visible above the fold), and first bit of the article content.
We don't control the speed (roundtrip latency) with which users can connect to our servers (e.g. via cellular radios and cable), and we also don't control their bandwidth (e.g. how many KB/s). However, it is important to remember that bandwidth is not speed. Transferring 100KB of data with 10 ms latency takes essentially the same amount of time on a 400 KB/s connection as on a 100 MB/s connection. The bandwidth controls how large individual packets can be (at most). It does not control how fast they travel.
The size of each chunk of data is automatically determined by TCP Congestion Control and related algorithms. However, for historical reasons, the default for most browsers and operating systems is to send/accept upto 10 packets of 1432 bytes at once. After that initial burst, the algorithms take over and grow the chunk size with each roundtrip until it finds that it cannot send more. This fluctuates and bounces over time. But, this is why 14 KB is a common target because that's roughly the amount of data all users receive at first, regardless of how high or low their bandwidth and latencies are (2G, 4G, WiFi, etc.)
- The 14KB In The TCP Initial Window, by @thcipriani (2016).
- PageSpeed Insights docs, by Google (2018).
- Optimizing for Maximum Web Performance, by Cloudfare (2012).
- T104859: Verify our current value of initial TCP congestion window