So it's pretty funny:
Since older iPhones have batteries that don't hold up well anymore (the batteries are glued in, so you can't replace them yourself), Apple decided to slow down the phones by lowering the CPU frequency. The phones are less responsive, slower, but consume less battery.
In France, Apple took a lawsuit, lost it, and was forced to give the iPhone full performance.
As a result, all over the world, some iPhone users change the settings to "France" in order to have better performances.

EDIT: I've been told that since some versions of iOS, the setting is available. So no need to change for France: https://www.theverge.com/2018/2/7/16984234/how-to-iphone-throttling-ios-11-3

(from https://sebsauvage.net/links/?5i7aNA)