Let's start with the last thing: It is close to constant speed - as pointed out in the article, the more data sets you have, the longer it takes to download and aggregate the entity. But the search itself is always the same steps and therefore constant. Obviously, having an entity with 10.000 data sets in it will not load in 150ms - but finding the place where the full data is stored is easily doable in that time.

I agree, that having a server which you bought once (or rented) is predictable cost. But what happens in cases of burst? You would have to buy another server (without knowing if you will still need that tomorrow). Predictable in this case means, that since you have the same steps for each request, you can tell the exact the cost per request. And that is from my point what is important.