Why do all iPhones slow down charging after 90% ?

It is the charging profile of the phone. They charge fast until about 80% and then the charging process slows down

A lot of portable devices work that way...charging does work on the battery and produces heat, slowing the charging rate allows the battery to cool a little and take on more charge. Also, the risk of overcharging is reduced, which the charging circuit is designed to prevent. So the slow down reduces heat load, makes for a more accurate charge level, and as a battery reaches full charge the energy input takes a little more work to accomplish.

Sort of like a charger for you car battery. It charges quickly while the battery is flat and and when the battery nears full charge the charger will go into trickle charge mode and the charging process slows down