I was using a Sony CP-AD2 2.1A charger with my iPhone 7 Plus and, according to the app "Battery Life", it was supplying a maximum of 1A only.
Also, I didn't notice any difference between the Sony charger and the stock 1A Apple charger.
So, I've bought a 10W iPad charger and now the app is recognizing it as a 2.1A charger. I've also noticed the phone is charging faster now (and getting a little bit warmer).
Maybe the iOS doesn't accept the Sony charger as a 2.1A charger and only asks for 1A from it.