Using an iPhone for too long can cause the battery to age, resulting in slow charging. In general, the life of the battery of the ordinary mobile phone is between one and three years, the mobile phone is used for a long time, the mobile phone battery performance deteriorates, resulting in the phenomenon of slow charging of the Apple charger.
When charging the surrounding environment, the phenomenon of slow charging will appear, the temperature has a certain impact on the charging speed, the ambient temperature is too high or too low will cause the Apple charger to charge slowly.
That answer applies to most of Apple's phone models.