The correct option is A better
The given sentence is an example of the adverb of comparative degree. An adverb that compares two things becomes a comparative degree of an adverb. 'Slowly' is an adverb of positive degree. 'Well' is an adverb of manner/degree. 'Very fast' is an adverb of manner. 'Better' is a comparative degree of an adverb. Thus, option A is the correct answer.