I feel like I’ve seen half a dozen HN submissions about this 900 WPM speed reading "barrier", and I’ll say it again: color me unconvinced until a proper study is done that measures people’s WPM alongside both their short-term and long-term recall.
Anecdotally, my ability to remember what I’ve read seems directly proportional to how much time I spend quietly reflecting on each section of a paper.
Speed reading seems more like a parlour trick than anything else.