From a physics point of view aerodynamic drag varies in relation to the square of velocity at "low" speed, and in relation to the cube of velocity at "high" speed. Exactly what defines "low" and "high" speed depends on the profile of the object. So all things being equal, 30% free effort at "low" speed might get you something like 5-6 kph, and only about 3kph at "high" speed. That's why it's so hard to go fast.

Of course I'm talking purely based on theoretical physics here. There are some parts on order for my first road bike... I should get to pick it up next week.