Yeah, I wrote out ~4000 words longhand and filled a whole small notebook for a personal project earlier this month...but it took me a couple of days and a lot of long pauses to think about what I was writing.
It's going to be a case by case scenario. Not everyone is at the same level of writing or able to arrange their thoughts coherently in a timely matter at all, especially given that most people's experience with writing tends to be in incredibly short concise bursts. (e.g., twitter, Facebook, reddit, with the latter having an unofficial max length before you get a tl;dr from most readers).
Some writers are like you; they take their time, think about what they want to say, then they say it. Others tend to be burst writers, where a few thousand words appear in the editor under an hour.
My experience as a TA in university is that a lot people who are otherwise perfectly competent communicators are poor writers simply because they're afraid to commit words to paper. They overthink how they want to convey their ideas or aren't sure how to logically arrange it and spend more time mentally planning and never committing something. It's probably why there's so much dread with paper writing even at the University level, especially given that many courses even within the sciences put a lot of weight on the reports as opposed to the work done. (This, admittedly, comes second hand from observing my roommates and g/f deal with Biology, Chemistry, and Physics papers and getting frustrated because of how heavily weighted the papers were on the class rubric)
The advantage to something like this, if you can muster up the courage to just write, is that it gets you used to just being fine with whatever you put out. It's not necessarily about quality as much as it is quantity and getting you used to putting out bad work and not letting that fear be crippling. I had exercises like this in High School where we were given journals and 50 minutes to respond to a long prompt on the given book we had read. They were difficult exercises, but well worth the effort.
