I've been reading (and writing) considerable amounts, pretty much every day all my life since I was 12-ish (and ~15), 37 as I speak. I'm cognitively no genius by any metric but certainly 'well capable' relatively to most people (strong points being speed of thought, clarity/precision to a fault, powerful synthesis). I however have a bad factual memory in general, I need to 'do' or 'understand' things to really memorize (e.g. I suck at poems, lyrics, can't even remember mine, but I love systems, physics, logic).

Without explanations or hazardous 'hypotheses' (hum) as to why, here are my observations and personal 'tricks'. Spoiler: no magic, just directed effort, solving for X.

- There is an inverse relationship between speed and depth of information input. The faster you take it in, even if you feel like you understand all of it (and may, factually), the shallower your actual intake will be in the long run. Generally. Emotions act as a catalyzer for memory, but cloud thinking temporarily.

- The more massive a piece of content (book, article..), the slower I want to go if I aim at a constant result (comprehension, food for thought, memory, etc). I may speed up when cruising if the content lends itself to that (repetitive, very structured, etc). Essentially, whenever recent memory won't cut it to think and remember, because there's too much content, or it's too complex, I need my long-term memory sparked into activation.

- I read fast in general, but it has nothing to do with 'techniques' related to the act of reading itself: it's entirely a mental 'mode' or 'state', that I only ever get otherwise in sports, this moment when you just trigger the beast and commit entirely to victory. When you just won't fail, when it's 'all-in'. That level of focus, of concentration, is best exercised with training, of any kind. So indeed you need to think, not just read, and actually want to even write and use the substance if you are to maximize its results on you.

- It's very important for me to begin slow, whatever the average pace later on. I need to get acquainted with the author, their go-to vocabulary and grammar subsets, the point they're making. Sometimes it never clicks and the whole piece is tedious for me. Sometimes it's so clear I feel like I could have written it myself, at least the structure of it. It's a thing, hard to explain. I guess why all we have our own favorites and dreaded authors.

I like people who speak relatively fast because I don't get easily distracted or bored when they talk to me, so speeding up audiobooks is a personal treat; but there's a limit if I want to retain anything more than fuzzy snapshots of partial points. I may understand 100% on the spot (I actually do nothing else but listen and focus), but it's evading memory, evaporating in the long run. Ultimately it means that book won't change me beyond a few impressions, it just won't make its mark, it never had a chance to imbue my consciousness — only my superficial cognition. However sharp, it can't substitute for the real thing.

An exercise I do if I really want to use or remember (be 'changed') by some piece of content — article, book, chapter, quote, podcast, piece of code... — is to do the "good student duty" of writing a short essay: summarize what you read, raise a few interesting points about it, elaborate a bit beyond (make that new material become food for your thought, use it like it's a new word you just learned, or a new math technique).

It only takes ~30 minutes or so, easily 10 minutes if the material is easy, an hour if it blew your mind, I don't know... well worth it, because it has a lasting power for years. But do it well, because that exercise you 'submit' to no one but yourself will become part of the memory of that content, you will see your own words sometimes before/above the actual author's.

Hopefully this helps someone trying it my way. The hard way. The no-genius no-magic no-trick way. The way that lets one play in the present with stuff learned 10, 20 years before.

TL;DR: the faster you read, the shallower it gets beyond some sweet spot, probably biological but trainable; speed is good when it forces you to focus, but not beyond that — it must not become a distraction from actual understanding. If that is your goal. Using at least in thought what you read is my preferred way to really internalize something, make it mine, thus remember it for long enough usually. At least I probably won't be able to forget that it exists, and then I can search it for finer points.