Why battlecruisers and battleships favored different gun weights

Were the different gun calibres proposed for the G3 battlecruisers and N3 battleships a safeguard against slow rates of fire?

The difference reflected mission and target set: battlecruisers needed useful fire against faster, smaller ships, while battleships prioritized the heaviest possible blow against enemy capital ships.

The calibre comparison starts with the targets each capital ship is expected to fight. Fast reconnaissance actions against cruisers reward a higher rate of fire, while a battleship fighting another battleship needs a heavy shell against a less nimble target. The 16.5-inch weapon discussed here is treated as a compromise near the upper limit for engaging a broad range of ships; an 18-inch gun sacrifices more firing speed for weight of shell. As main guns grow beyond 12 inches, destroyers and light cruisers become increasingly difficult targets for them. A six-inch secondary battery can take over those engagements and is described as the largest calibre that can still be manually loaded. This explains why the main and secondary batteries have to be considered together.