This means different things for different people. Some can read with quiet music playing, others have no problems sitting on a park bench next to a noisy playground. You should realize that you can read different types of material at different speeds and still be reading both efficiently. Your Biology textbook may slow you down a lot more than the suspense novel you got out of the library. New concepts and technical detail will take more time for your mind to absorb than will easy vocabulary and a familiar subject.
Some people have the bad habit of reading aloud silently.
To find out if you are one of these, put your finger on your lips while you read. If you feel your lips moving, you are subconsciously sounding out every word you read. Since we all talk slower than we read, this will always slow you down. An even more common problem is re-reading lines or parts of lines. Sometimes this is necessary if the whole meaning is not understood.