Readers of this space learned a few weeks ago about Spritz, an app that promises to dramatically increase your reading speed by converting text to a fast-moving sequence of individual words or phrases. Because the text is moving, your eyeballs don’t have to, so you get through words faster. Such speed-reading apps have been getting a lot of attention, but a new study published in the journal Psychological Science warns of a downside: You may read faster but learn less.

In “Don’t Believe What You Read (Only Once),” researchers from the University of California at San Diego say that the apps’ manipulation of text denies readers the opportunity to “regress” — that is, go back and reread a word or sentence — and this had a demonstrable negative effect on comprehension.

For the study, 40 students were asked to read text on a computer screen, and their eye movements were tracked. Sometimes the text was presented normally. Other times words were masked as soon as the reader’s eyes moved on. Testing the students on what they had read, researchers found comprehension was impaired when the students couldn’t backtrack, whether the masked sentences were simple or ambiguous.

“Our ability to control the timing and sequence of how we intake information about the text is important for comprehension,” the researchers concluded.