The first principle of speed on the classical guitar is the fact that whole pieces are not fast. Speed in compositions comes in bursts. This is the basic tenet of comparison. A piece played Largo might have Andante passages that are fast. But these Andante passages are slow if compared to a composition that is presto or prestissimo. So be aware that speed is not necessarily how many notes a second but more a factor of notes and phrases in comparison to each other.
Practice, Practice, Practice
You already know that practice is critical to speed. You have to practice your scales just for the practice, and you have to practice a wide variety of scales. Classical Guitar is like every other form of kinesthetic art; as you practice the motions you make will settle into deeper regions of your brain and your body will learn how to do it without you even thinking about it. With practice you will be able to blast out very fast scales that will amaze you.
Now, all of that sounds good but what about some practical advice on how to get faster?
This part is easy, and the single best thing you can do to improve your speed is to make a conscious attempt at finger crossing patterns with your right hand. This is usually the biggest challenge to playing speed. Practice, on a daily basis if possible, crossing string patterns.
What are string crossing patterns?
This is the way you pick across the six strings with your right hand. If you are playing a scale and you transition from string to string with the right hand you will use a pattern such as playing the first string with your index finger then playing the second string with your middle finger. On to the third string you are back to your index finger and for the fourth string again back to the middle finger.
Avoid the same crossing patterns and create new ones
As you become aware of your finger crossing you will see that you have very distinct patterns that you use. You should create and practice new patterns that are not comfortable for you. This truly will dramatically increase your speed. A good example of a new pattern you might try is to switch your starting finger. When practicing scales you probably start the first note with your right hand index finger. And as you progress through the scale you cross scales in the same pattern. You should try starting the scale with your middle finger. This will totally change the crossing pattern you use for playing the scale and once you get a bit of practice like this your speed will increase significantly. Vary this crossing pattern in as many ways as you can and make sure you also do double strikes where you cross using the same finger.
To improve your playing speed on the classical guitar you have to practice and you have to bump yourself out of your normal routines of playing. But with some conscious effort you can significantly improve your speed.