Where does the difference in car performance come from?

I often hear on TV during IndyCar race that such and such cars are very fast today. Where does this performance difference come from? Is this from...

Source: http://www.motorsportforums.com/indycar/146635-where-does-difference-car-performance-come.html

Tadahiko Taira Kunimitsu Takahashi Yuki Takahashi