Well, we were taught to “read slowly and carefully” which made it easy for us to go back over words repeatedly. This is called Regression or back-skipping if you like.  This not only slows you down, it causes you to have difficulty with comprehension. For instance, let’s say you have a sentence, "The man jumped over the log." Well, if you back-skip, you read that passage like this: "The man jumped," "the man . . . jumped . . . over the log," "jumped over the log." So, what your brain is processing, "The man jumped," "the man jumped," "jumped over the log." Our brain is used to processing our flaws, so the brain thinks, "OK, I know what this clown is saying, "The man jumped over the log." But this takes time to sort out. And it's confusing. Think how much easier it would be if you simply took the sentence in one sight, "The man jumped over the log." There's no confusion there. Then you move on to the next phrase. Regressing or back-skipping is the most harmful thing we do to slow our reading speed.