With most modern smartphones you may have likely noticed a notification message while charging your phone indicating that your phone has entered "Optimized Battery Charging", this is how Apple refers to its smart charging messaging. With Android devices they will also show this notification message while charging although they refer to it "Adaptive Charging". Through these intelligent charging methods your phone will artificially reduce its charging speeds so that in the long run it will lengthen its overall battery life.
When setting up a new smart phone it may take a few weeks of machine learning to recognize your usage behavior and charging habits. Batteries go through wear and tear to which the more charge cycles it goes through the less healthier the overall battery becomes leading to it holding a lesser charge and other issues.
Your phone will alert you with a notification when it enters into a battery preserving charging state. At night you may notice your phone charging significantly slower along with a message saying “Battery full by 7:00 AM”. Even though it has the ability to fully charge in a much shorter time. If you set an alarm for an even earlier time your phone can adjust to finish charging at your alarm time. This is your phone prolonging the time it takes to charge to 100% in order to preserve battery health.
The reason why your phone will artificially slow its charging rate is to spend less time at 100% battery, and the less time your phone spends at 100% the more it helps with your overall battery health.
It is possible to disable Optimized/Adaptive Charging in your device's battery and charging settings although it is ideal for the majority of users to leave this setting enabled.