First, the [lithium-ion polymer batteries] used in the iPhone\/iPad have a higher power density. A lightweight battery can provide a long battery life. Lithium-ion polymer batteries can also be charged at any time without the need for a full charge and discharge. For the iPhone, the first 80% uses fast charging, which takes less than 2 hours to full the first 80%, which is why it has started charging so fast.
After that, continuous supplementary charging is used, which is slower, which is a trade-off for battery life and use.
If you want to speed up, turn on the flight mode charge, a tube of electricity will be faster than usual for 30 minutes, (if you go to climb a mountain, when you climb to the top of the speed will be slower than when you just climbed, although the mobile phone charging is different, but to a certain amount will produce reverse pressure, resulting in a slow down) for reference only.
Because it is likely that there is a problem with your battery, it can also be a problem with the charger