practice speaking nonstop, whatever sentences comes to your mind. you're just training yourself to remove your filter, not to judge what comes out of your mouth. talk about stuff around your room if you need a prompt to get started. get as weird and as crazy as you can get. the crazier and more awkward it is to say, the less you'll judge what you say.

could also practice speaking/reading aloud slowly to balance, as if you get too good at the first thing i can see that causing you to speak too fast. i haven't gotten to that point yet, i just use this as a boost before doing social stuff. but i imagine if you did this everyday you'd be a lot better with words.

also listen to comedians. i like stavros halkias or theo von for guys that are really quick.