My older brother is a really fast talker.
Why did you say that you love me the most so quickly?
I’ve never seen such a fast talking baby before.
When you have a fast mouth, you're probably a fast talker, right motormouth?
Remember how body parts often change even jukugo words to kun'yomi readings? That's what happened here too. Both readings are kun'yomi, making this one はやくち.