You don't have to read particularly fast to score high in GMAT verbal, partly because most of what you have to read is not particularly voluminous.
You do have to be quick in other ways though. You have to be quick to pick up on the logic of what's stated, to notice details that are key for determining which choice is correct, and to see whether something written makes sense.
The way to develop those skills is, of all things, to practice slowly and carefully. In fact, one of the most common mistakes made by people training for GMAT verbal is seeking to answer practice questions too quickly. You won't learn to do what you have to do if you don't give yourself time to learn it. So, when training, you have to give yourself all the time you need in order to see what's going on in questions. As you develop skill in seeing what you have to see and in clearly defining why choices are incorrect or correct, you'll speed up.