In my experience, it's the query time. Maybe it should be faster, but practically, with all the tech debt that goes into these things, it's not. I've actually experimented with different sizes for a social media app (meaning it queries total reacts, shares, etc) and 15 was close to optimal.

Latency is not an issue, and neither is displaying complex information. You can pull and display 400 dummy items no problem.

Loading time for the first "page" is extra valuable - you'd want that in 200 ms or so ideally. So one trick is load 3 items, then 30 or so.

Also you have to look at actual cost. Perhaps loading an extra 15 queries on the home screen costs $0.0004, but when you have 1m daily active home page uses, that's an extra $400 per day. In unoptimized pieces of code, the cost could well be 20x higher.

If you have a very high average user value like Jira, that's fine. But for say, a free manga site or something like imdb, you want to shave off costs wherever possible.