It depends on the number of charge/discharge cycles, plus presumably some manufacturing variation on the batteries they source.
Someone who is a heavy phone user who runs it near empty every day is going to age the phone a lot faster than a more moderate user. I usually charge my phone every other day so the battery in my 25 1/2 month old 6S plus I traded in last month was in the same shape as a heavy user's battery would be after only one year.
As for a "sliding scale" it sounds like they knock down the speed the weaker the battery gets, so it isn't a matter of this model gets X% slowdown and this gets Y%. If you read Poole's article there are multiple peaks shown for the same model, which represent greater and greater amounts of battery wear.