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I've read about studies where they quantified how much was retained between people reading "normal" speed, and those speed reading after taking a course, and it didn't bode well for those "super memory" type courses that end up being just (surprisingly unrelated) speed reading. At first they seemed to have learned the material, but only a short time later it was only the "slow" readers that passed the same tests. And the slow readers had more complete recollections, resulting in richer descriptions of what they had read.
My take (which could be wrong) is that speed reading makes it easier to convince you that you suddenly have "mega memory" because you could apparently read faster and retain more information than you thought you would have been able to, but really only until the end of the testing period. They're much like other snake oil treatments such as anti-wrinkle creams that actually just puff you up with fats and lymph - the wrinkles are gone, but it's a mirage of sorts.
There are many of them known. Sometimes mentalists use knowledge of them in their tricks. As do "faith healers." Miraculous changes that end up only short term quirks of psychology.
How many people do you think have read Tony Robins' books and got super rich like he supposedly did using those same techniques? You'll apparently feel really confident while you still live the way you were otherwise, and blame every positive event that would have happened anyway on that book.
Snake oil. But good if you want to learn how to skim (or get rich quick, since that's usually the supposed end point any time such programs make it to TV).
It's slower than speed-reading, but my ears are available for 2-3 hours per day, so I end up "reading" a lot more with this strategy.