I was fortunate to be given a speed reading class that took several weeks as a high school graduation present back in the dark ages (1965). I took the course and practiced as I was told, but I never did believe it was doing any good and it was definitely not a fun way to read anything for pleasure. Fast forward to the next year and I am a freshman in engineering, but having to take an "elective" political science course in which I had no interest at all. Since the purpose of the freshman year in engineering is to cull the masses I concentrated on what, to me, were the far more important classes I had. A few days before the final exam I realized that I was not going to pass that polysci class unless I could pull off a near perfect score on the final. The professor was kind enough to confirm my calculations on that point. For two days before the final I read the entire text book through cover to cover TWICE. I scored in the high 90's on the final and passed the course with a C+. When I was taking the exam, I really was just sort of zoned out. Much of the test was essay questions and I would just write whatever came into my head on the subject, not really knowing what I was saying or where it came from.
Now, I would have put this down as a fluke except that I was able to do very similar things for the rest of my career in engineering. Although trained in systems engineering I started out working with computer systems when computers were big iron and I worked on both IBM, Univac, and DEC systems. Then I successfully made the transition to PC's and networks and retired as a Network Engineer and Security Officer. Often I would have to learn enough to get started in a new area about which I knew almost nothing with little time to do so. I would get 5 or 6 books on the subject and absorb them over a weekend and could then get up to speed pretty quickly after that.
I still don't understand how it works, and I am still not sure I really believe it works, but for over 40 years the speed reading class I took in 1965 saved my bacon many times.