This is one of those things that, as a nerd, I assume everyone knows, but the reality is quite the opposite. While your iPhone obviously came with a wall adapter, that wall charger doesn’t charge your phone as fast as possible, as it it is a 5-Watt adapter.  If you want to (roughly) double your  charging speed, use a 10-Watt or 12-Watt iPad adapter. It’s perfectly safe (at least as long as you use an official one), and I think you’ll be pleasantly surprised by the difference in charging speed compared to what you get out-of-the-box.