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No, not necessarily, but it can cause a minor issue. iPhones use a Lithium-Ion battery for its power, using while it's charging won't necessarily ruin the battery, but if you're using the phone while the battery is in the charging process, it can cause the phone to think it has more power than it actually has.
Only charge fast when you need to. A lot of newer phones support some sort of fast charging feature. These suckers will let you juice your phone up from zero to around half-full in just about a half hour. It's a life-saver for when you've only got a few minutes to spare, but it's also not great for you battery. Surprise!