Why does the last percentage of my iphone battery takes so long to charge?

I have noticed that the very first percentages (1-20) of the iphone battery charges very quickly, but when it comes to the last percentage (99-100) it takes like 15 minutes to charge!

When you first got it you probably charged it all night well they only need about 2 hours to charge so the over Douce of charge now makes it slower to charge try to get a new battery or a new phone.