Water boils at 100^o C at 1 atmospheric pressure. So long as trace of water in liquid state is there, a constant temperature of 100^o is maintained in the container. Cooking involves hydrolysis of certain ingredients of food (e.g. pasta) through application of heat. The speed of such hydrolysis is directly proportional to the applied temperature.
Consequently, while cooking pasta, it is immaterial whether the wate is boiled slowly or rapidly. Rapid boiling means rapid rate of evaporation, temperature is maintained at 100^o C in both the cases. Pasta will cook in the same time in either of the situations.