How do you learn to stop subvocalizing...that's what I always have trouble with.
You may have just the comprehension glitch with the speed reading.
I know that it's a scientific fact that if you take in information both Visually and and Audibly you will understand the information better and remember it for a longer period of time.
So hearing your "inner" voice while you read may be what hleps to retain the information longer and eliminating this part of reading may be why I and other have found shorter retension times with speed reading since the inner voice is suppose to be silenced.
I've watched and read the proclimations of Speed Reading saying that comprehension went up, but I've never seen even one of talk about HOW LONG the information is retained or the percentage of the retension.
They talk of testing people for comprehension after speed reading, but I'm asuming that it's within about an hour of when the reading was finished.
I highly doubt that they tested them 6 months to a year after the material had been read.
As stated in my above post, at my SLOW 350 WPM using my inner voice I remember around 80% of what I read for YEARS after I read it. But when I speed read, I'd say that I remember maybe 40% of what I've read after only a week has gone by.
P.S. If you want to see what it's like to read WITHOUT subvocalization just SKIM over the words as fast as your eyes can move while still keeping the words IN FOCUS. You'll find/feel that your eyes will slow down when you try to automatically subvocalize.
You wind up looking at words the way you view PICTURES. When you look an IMAGE with no letters you don't subvocalize. This is another reason that some Speed Reading techniques use image viewing to help train you to read faster.
Some also teach pattern recognition. Kinda like this:
"Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe"
MRC CBU, Cambridge » Matt Davis