I also struggle with reading speed, and I can easily to get lost when reading dense material, but what also helps me stay focused is actively annotating my thoughts on the page while I'm reading.
I mean, brain is just a set of chemical reactions based on inputs. Reading has 3 parts. (1) Eye running through text and sending it to the brain. (2) Brain is trying to connect the content with other neurons. (3) Storing data, probably in the form of neuron connection, after step (2), which found correct place to store.
Now assuming process never ends and human anyway can't control 100% of that chemical reaction. While your brain is trying to figure out other thing, can it still execute steps (2) and (3)? Because (1) didn't stop
My brain can not store sence if the sence is exposed to me. If the sence is yet to be exposed this is GIGO.
I think visual cortex may remember visual information - pictures, funny fonts, position of some lines of text, maybe some specific words even if the book is foreign and not understandable. But I do not believe you can cut off all words out of book and read it in random order and receive understanding.
BUT from my experience of reading, there is one way of conscious reading without reading the page fully - when I am reading super quickly for finding "good" and "bad" words. In my scientific areas there is often enough to see or not to see some specific words and surnames to understand what approach is being describe and is it novel for me. So, I believe there is a JS programmer who can read a totally new 200-page book "JS for beginners" for less than 5h and understand it fully.