because we need quadratic energy increase to increase speed linearly, that's why a 200hp car is not twice as fast as a 100hp one. Gee let me elaborate a bit more... if the 100hp car has a top speed of 100mph, a 200hp car will have maybe 130mph of top speed (assuming all other things the same) because you are fighting friction of an inmense amount of things that want to stop that movement. Anecdote... this famous US plane made with this titanium allow, I remember reading that at the speeds it was able to reach, the pressure of the air hitting the surfaces is so much that it will cause other metals to fail. Imagine the amount of energy you have to spend to heat the whole surface of the plane while traveling through cool air!