Face ID on Apple's newest iPhones is already really fast, but it requires that you look directly at the phone to unlock it. Even then, it sometimes won't recognize if you're wearing certain types of sunglasses. This adds an extra level of security to Face ID.
You can remove this security feature to make Face ID even faster and more convenient by deactivating a default option that requires your attention.
Your iPhone is still secure and Face ID still requires your face to unlock, but it gives you a little wiggle room to wear sunglasses if you've had trouble using them with Face ID. It also makes things a bit faster if you're doing something else, like talking to someone or looking at a computer screen.
Here's how to tweak Face ID so that your iPhone can recognize you even when you're not looking at it.
There's a small note here that says "TrueDepth camera will provide an additional level of security by verifying that you are looking at iPhone before unlocking. Some sunglasses may block attention detection."
So, again, this is a fair warning that you're making your iPhone slightly less secure since you don't need to look at it, but your face still needs to be in front of it. It's still secure. The Face ID sensor still verifies the 3D shape of your face, but now it'll work better when you're wearing sunglasses or not looking directly at your iPhone.