Why lighter guns did little to speed up a 74

Could a two-decker become an Age of Sail battlecruiser by carrying many lighter, frigate-sized guns?

Uniform 18-pounder batteries were tried, but they did little for speed; razing away a deck was more effective.

Some 74s were tried with a uniform 18-pounder armament, offering a cheaper third rate. Replacing heavier lower-deck guns with 18-pounders did not meaningfully improve their sailing speed because the hull still relied on the same wind power and sail area. Razing the ship removed an entire deck and produced a much larger reduction in weight. The resulting frigate retained much of a third rate's framing, mast strength, and sail-carrying capacity.