With Face ID on the iPhone X, it took 1.2 seconds from pressing the side button to the iPhone X's screen turning on and for the phone to recognize me and unlock the device. And it was another 0.4 seconds to swipe up to get to the lock screen. Total time: 1.8 seconds. (You can simply tap the screen to wake it, but this took about the same amount of time).
On my iPhone 7 Plus, I could get to the home screen just by pressing and holding my thumb on the Touch ID sensor in an average of 0.91 seconds. That might not seem like a lot of time, but it adds up quickly when you're unlocking your phone dozens of time a day.
Across a whole day, this could add up to literally tens of seconds..