When you are listening to people speak in a language that you don't now, it seems like they are talking so fast, but after you learn that language, why is it like everyone is now talking slowly?
When you are starting to learn the language, and you hear someone speaking it, you try to understand each word separadetly, so if someone speaks "Hola, yo cocino pescado" (random phrase), when you finally understand "Hola", he just finished the phrase, making it seems that he is speaking fast.
It's because before, it was just sound. Now you can distinguish and recognize what those sounds mean. And as you get to know the sounds better as they imprint further onto your brain, the faster your brain recognizes and retrieves them. So there is less of a delay in your brain processing what you are hearing.
Because the people you hear speaking the different language know their language inside and out, when we are just beginning to learn it. They may have grown up with that language, and notice how you speak in your language? Fast. That's how it is with them. Hope this helps!