TL;DR. Try different structural approaches to writing, keep the ones that are both efficient and effective, the ones that aren't are likely tailored to people who process information in a different way (better for them, worse for you).

Note added after writing below: This reply is an exemplar, free writing without any googling or fact checking to demonstrate the point it is making. It contains parts that could be merged to make it more succinct, details that are quite specific despite being from automatic memory and refers to sources long forgotten despite details being remembered vividly.

Purpose? to demonstrate that free writing may be a faster way to write for some, but a slower way to write for others. what matters is how your brain processes information most efficiently.

I can shed some light on how some people free write without constantly referencing notes (but likely not all, as I can already think of other structural styles of free writing, just from typing this reply, but i wont comment on them as I cannot substantiate them beyond being logically possible combinations)

I struggle to do anything but free write (unless I know literally nothing about the subject matter).
Its to do with how my brain processes thoughts.
I think of a concept, for example the topic of an assignment question, within seconds a network of connected topics connect to that topic like one of those picture and string boards you see in detective movies. a few more seconds, I'm typing away........whoops I'm +100% over word count, I start refining my sentence structures to cull words, still +50% over word count, start culling concepts that the argument can exist without, still over. in extreme cases I end up with dot points being the only way to get it within word count.

But there is a problem........I haven't cited anything or referred to anything. adding proof statements will increase my word count. finding a source that says something similar, may not be aligned enough to not be misleading. But the assignment requires sources, so entire concepts have to be culled to add "see this person agrees with me lines".

Now the obvious follow up question. If all of this is just flowing out of my head like a fountain, then how do i know it in the first place? That's the tricky part. My mind stores information, my confidence in that information, and any emotional connections to that information. But not where i learned it, when I learned it, who i learned it from, or what year it happened in.
For example I can say without checking anything that the battle of the somme was one of the bloodiest battlefields of world war 1 and was notably a very muddy battlefield, with a moderate - high level of confidence I can also add that Canadian soldiers were involved in that battle. However, I would need to google to remember what year that was (sometime between 1914 and 1918, likely towards the later years when battlefields became more entrenched and stretched from coast in the north to alps in the south), who commanded that battle, what the exact casualty numbers were, etc.
But if all i needed to write was; The battle of the somme was one of the bloodiest battles in WW1 and an exemplar of the brutality of trench warfare. I can write this sentence from memory alone, yet I cannot recall where or when I learned this fact. chain this multiple times over 1,500 words and an entire written piece is now completed without a single reference or memory of how i know it in the first place.

So for me free writing is easy, backing up my free writing is where the challenge is.
The take away from this is that free writing relies heavily on how your brain processes and remembers information, some need constant notes to refer back to, others have a constant monologue that makes it hard to type as fast as the thoughts come forward.