While quickest can function as an adverb, "He did it quickest", it's forcing a superlative adjective into that role. What "He did it quickest" is saying is that "He was [the] quickest doing it". It's a colloquialism.
This is why it sounds awkward in your complex sentence: a more complicated sentence needs a more rigorously "correct" form which actually uses a true adverb, most quickly.
It's also A Good Thing to put the adverb closer to the verb it modifies.
Choose a specific process that would most quickly create the most impact.
Changing the order can subtly change the emphasis; in your original the emphasis is on speed, whereas in my example here it's on the impact. However, since speed and impact are both to be maximised, it's a distinction without a difference.