Why does the speaker say "faster" not "fast"?
Answer from a Native speaker
Remember that "faster" is a comparative adjective. While "fast" is used to describe things or people that are moving quickly, "faster" is used to indicate comparison. Thus, we can understand the baby as having used "faster" to indicate a speed that is faster than the current speed. He is using "faster" to urge them both to suck on their pacifiers faster. Ex: The red car was faster than all the others. It was clearly in a league of its own. Ex: Faster! You're driving too slowly.