For me at least, reading speed inversely correlates with reading retention. There are plenty of non-fiction books that are about 20% useful information, and 80% anecdotes in support of that information.
If I find that I’m already convinced of the point that a particular subchapter / section is trying to make, I’ll speed through it. As a result, I speed read through about 80% of most non-fiction books.
My experience with speed reading is that it’s more akin to speed “skimming”. I see all the words, I understand the point each paragraph is making, but I’m not resting and respecting every word, or really paying too much attention to sentence structure. You can miss details, but this is predicated on the assumption that those details don’t matter.
If I’m reading something where every detail does matter, or fiction that’s heavy on prose, I slow down significantly, since my objective is often to enjoy the book for the maximum amount of time possible, and not to learn as many things in as short a time as possible.
Often it is required to cut off some time to read documentation and googling. Most of the time the content has 80% information that I don't need at the time (they're important, but not right now) so I just skim through parts.
When I get to the parts I need, usually I skim it once and reread it slowly again after.
The way i read is that i usually build a synthesis as i read. A lot of sentences exist not to convey a particular point but to build upon or around and existing one. If you focus on reading while building this "structure", then you can read them as patters and you do not need to remember every single sentence.
If I read a book, i will remember at the first reading nearly all the points, the structure, what it tries to talk about, what i like and why. I will not remember all the details of every single arguments or all the details of all the plot points. It is a different way to read and goal.
Speed reader usually sacrifices some details during reading, skipping conjunction is one of the example. So yes you need try to learn fast reading.