It doesn't need it, folks. It's a static blog page.
Instead, every x seconds it executes another POST request with pretty much all the details they can gather (scroll from top, scrollable height, referrer etc.).
As soon as you start moving your cursor, the new requests start adding up very quickly, with lots of new params such as "experimentName: readers.experimentShareWidget" or "key: post.streamScrolled".
It really is collecting every single interaction with this page. As it's provided by Medium I'm sure it's part of their data collection program.
Just because it can be cached doesn't mean it's free or even necessary.
Which kind of puts the opening paragraph into question:
> I’m passionate about image performance optimisation and making images load fast on the web.
I've noticed the word "passionate" gets thrown around a lot cheaply.
It's seriously starting to lose all meaning.