I have the opposite problem. I hover around 60 WPM when transcribing or copying, but I often pause mid-sentence or mid-paragraph to gather my thoughts for what to say next (probably more like how to word it correctly) when writing original content.
Maybe it's a lack of discipline or practice and I'd be able to form thoughts more quickly if I were required to, or maybe I can form thoughts quickly but my typing doesn't keep up and I lose track. That said, I do the stop-to-gather-thoughts thing when I speak as well so maybe I'm hopeless.
So, for me it is that I can not really decide "what to say next" until I have gotten "what I'm saying now" out of my head and onto the screen.
That is, if I can keep an even pace where I'm typing at roughly the speed I'm saying what I want to type, then I don't do this stutter that often. However, if I am constantly getting adrift in how fast I'm typing versus how far ahead I've thought of what to say, then I stutter like crazy.
I'm assuming it has a lot to do with how you "speak" in your head. Some people (myself) do so verbally. Some do so visually.
Maybe it's a lack of discipline or practice and I'd be able to form thoughts more quickly if I were required to, or maybe I can form thoughts quickly but my typing doesn't keep up and I lose track. That said, I do the stop-to-gather-thoughts thing when I speak as well so maybe I'm hopeless.