The problem

A running app has 5 million users. Lea finishes a 10 km run, and her 80 followers see "Lea ran 10 km" at the top of their feed.

A marathon champion on the same app has 1 million followers. When she posts her race, every one of them opens the app within the hour to see it.

Both posts go through the same feed code. Lea's 80 followers and the champion's million all expect the feed to open in under 200 ms.