As someone who tried using spreeder for a while, I can say that your retention is quite a bit lower than normal reading, especially when reading complicated texts, since you loose contextual clues (your brain is really good at doing look-aheads for clues).
It also seems to matter whether chunks are split around punctuation or not, i.e. If the sentence is about to end after two words, the current chunk should not show the first word of the next sentence, just show the last two words of the current sentence. I couldn't find a speed-reading app that did this the way I wanted, so I wrote one for iOS (http://velocireaderapp.com/)
I also agree with the comment below that dense texts are best read without speed-reading. There's a sweet spot between easy and hard, boring and fun for texts that are ideal for speed-reading IMO.
Is there any way to raise the speed above 900wpm without going all the way to 1200wpm? The book I'm reading now is fine at 900wpm (and <=3 words per screen), but I think I could go a little faster. At 1200wpm I'm losing comprehension, so ideally I'd like to set it to 1000wpm. I can't see a way to do that.
+/- buttons at the top-right of the screen to make +5%/-5% adjustments to the speed without stopping reading would be great.
I'll get back to reading...
Currently there isn't a way to do that without adjusting the number of words. I've had other users complain about this though, so in the next update I'm going to make the time slider free to move, and introduce a stepper to adjust to the nearest 50wpm or something. This update has taken a while because I've been working on Pocket/Instapaper support.
Originally the slider was free to move; it was just too finicky to get down to a round-figure WPM on the smaller devices so I added the 'ratchet'.
This took practice. I was reading economics books by Mises at 300 wpm in spreeder for a while and gradually increased the speed. I kept the speed low enough to understand it. However, it was never worse than regular reading.
Also, I started with 3 word chunks. I found that a lot easier than 1 word chunks. I normally use two word chunks now but that took work to get used to. I wanted to lower chunk size to reduce sub-vocalizing and eye movement. I've tried 1 word chunks but I lose over 100 wpm with them.
Anyway, low retention is caused by many things but is not inherent in RSVP.
When I tried the same site for reading blog content and short stories, however this method falls flat. I found it great for memorizing but not for absorbing or following a narrative.
If you use Spreeder, your eye still needs to move sometimes. That slows you down. Spritz has been carefully designed to ensure that your eye never needs to move, and never wants to move.
In my informal comparison tests, these improvements do make a difference.