If you want your iPhone to charge faster, anecdotal evidence suggests turning it off so it’s not using any power while it’s charging up will cut down charge times.
If you don’t want to hit the power button, then putting your handset in “Airplane Mode” (from the “Settings” menu) will stop your phone looking for cellular and Wi-Fi signals and can speed up the process.
At the very least, try not to use your phone while it’s on charge. Lock the screen by hitting the sleep/wake button on the top right of your handset to give it a break while it recharges.
Use a Wall Charger
Your iPhone will charge fastest from a power outlet, rather than via a USB port. Apple’s official advice “for the quickest charge” is to “connect the device to a power outlet using the USB cable that came with the device and an Apple USB power adapter”.