My personal guess is that, with fast typing speeds, it's too easy to just copy things word for word. With writing, you have to at least rephrase it and reorganize it to fit the notes reasonably on the page, which forces some processing to occur. I take notes solely by typing, but I only have retention if I do it slowly and reflectively.
In other words, we can "store" knowledge more efficiently when written in our own handwriting than when typed into a neutral/generic text editor.
Although, anecdotally, I find that writing notes by hand is better for recall than typing.
I'm not sure if this is because writing activates different parts of the brain, or simply because writing is slower and forces me to therefore think harder and comprehend better because I need to be 2x as selective about what makes it into my notes.
My money's on the latter, but who knows?
I can roughly keep up in real time with speaking on the keyboard. Even though speaking is a bit faster than my max rate around 90 wpm, by dropping filler words and gaps I can mostly avoid summarizing and quote verbatim. When transposing at full speed like that, I feel like a conduit at times, very little sticks deeply in memory.
Contrast with writing notes, where I'll only write down things I find particularly important. Most of the time I'm just trying to actively listen.
Purely anecdotally, I feel like when I am typing and summarizing to the same compression ratio as writing, my retention with typing is better, because I can do it without looking, it's faster, and I can tune back in to the speaker.
On the other other hand, with a laptop open, I'm much more likely to get distracted with emails/tasks.
Bottom line, I think we need to study more axes of this problem, if only because it gives neat insights into cognition.
For some reason I do not yet understand, the motion of physically putting pen(cil) to paper helps ingrain that information into your brain, in a way that typing it into a computer does not.