Silent reading is twice as fast as oral reading, but in order to become a better silent reader, you need to develop good fluency. Fluency is simply being able to recognize words automatically as you read, so that you can develop more expression in your reading and read smoothly with expression.
There are two common ways to improve fluency, the direct and indirect way. The direct method has a child read a passage of about 100 words at their reading level. The passage should be decodable but not predictable. The child will read and reread the passage with a certain amount of time until they are fluent. The indirect method encourages kids to read voluntarily in their free time.