Something which I never realised till very recently was that the iPhone and iPad chargers both have a different power output. The iPhone charger has an output of 5W which is suitable for its battery, but the iPad charger has a 10W output which is suitable for its large battery.
I've been using the iPhone charger as my main charger to charge both my iPhone and iPad and did notice that my iPad was taking longer to charge. I thought nothing of it because I've heard many stories that iPad batteries die slowly after a year of continuous use. I then realised that because the iPhone charger has half the output of the iPad charger, that it was taking double the time to charge. I switched back to my iPad charger and boom, my iPad was charging super quick, just like when I got it new.
If you have an iPad charger, use that as your main Apple product charger. Even though the output is higher than the iPhone's one, it won't damage your iPhone because it will only draw the amount of power needed, which is 5W. So it's better to have a high output charger, than a lower one.