Not sure what my typing speed is but I type pretty fast especially since I’ve been working/messing with computers for most of my life. I was taught the “proper” way when I was like in Jr High but honestly, I don’t know anyone who actually types that way lol my mom did I guess and she was an administrative assistant/receptionist most of her career. 
Anyway, I think it’s true what you say, your mind works faster than your fingers and you’re in such a hurry to get it out, especially if you have short term memory issues like I do (also a trait of ADHD amongst other things) because you’re trying to get the exact sentence/idea out how you envision it before you’re like…what the heck was I going to write again? Dang it! It was in my head a second ago! Gah! 
I think mental state can affect it too. It’s natural to make mistakes but even for work, I have days where my brain forgets how to spell something I KNOW how to spell, and I know that I KNOW how to spell the stupid word, and it’s like, I just have a brain fart. Or I keep misspelling the same word/fixing it in chats and I joke with my co-workers that I can’t English on those days.
Did you know there are people that exist that have literally NOTHING in their minds? Just blankness? Quietness? Like they’re literally incapable of envisioning anything in their head, might not even have internal monologue. When I learned that, I was like holy crap, that’s kind of scary. How do these people live life? But then again, having some silence in my head might be nice every now and then. Instead it’s a constant flow of different “TV channels” on at the same time. I think Sherlock Holmes has it right:
“You see,” he explained, “I consider that a man’s brain originally is like a little empty attic, and you have to stock it with such furniture as you choose. A fool takes in all the lumber of every sort that he comes across, so that the knowledge which might be useful to him gets crowded out, or at best is jumbled up with a lot of other things, so that he has difficulty laying his hands upon it. Now the skillful workman is very careful indeed as to what he takes into his brain-attic. He will have nothing but the tools which may help him in doing his work, but of these he has a large assortment, and all in the most perfect order. It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones.”
Our brains can only store so much and I think when we’re trying to remember everything/learn everything, especially with ADHD brain that wants to do ALL the things, the brain at some point is forced to put some things in the back burner even without us consciously realizing it. Some days it’s obvious when you can’t “English” like what happens to me, constant mistakes, can’t remember basic words or how to spell them, or your vocabulary suddenly shrinks to that of a grade school kid. Not even haha I have days where I’m sure a fifth grader can write better than me. 
