That's normal and it was specifically designed around it. However, there's no particular harm to using some a power supply that might charge slower, like 5W or 10.5W power adapter with a USB-A to Lightning cable if you feel more comfortable about it.
Most lithium-ion battery charging systems are designed around charging quickly to about 70-75%. Then it will taper off the charging rate (close to zero when full) since that's when it tends to get really hot if the charging rate isn't reduced. But there's nothing particular bad about using a power supply that will limit that charging current. The charging system will just use whatever it can.