If the track is bit curvy then it skips the lines and overshoots
Are you sure that it's lack of controller responsiveness that is the problem? Your system will have latency in many places: the sensors, the Arduino, the motor controllers, and the physical platform itself. Even if the first three items respond in effectively 0 time, would the physical platform be capable of responding fast enough to stay on the line? If your line is (say) 1cm wide, and it suddenly bends 45 degrees, your bot only has to travel 1.4cm forwards before it has passed totally over the line and is unable to recover. At 1m/s, that's a mere 14 milliseconds of physical movement.
If your physical platform can't react in time it doesn't matter how fast the controller is.
Practical Arduino: www.practicalarduino.com