Why an F3 battlecruiser cannot trade away underwater protection

Could an F3-style battlecruiser reach 28-29 knots by reducing protection, torpedoes, and secondary armament?

Technically yes, but the result would sacrifice capabilities and underwater survivability that the Admiralty still considered essential.

A narrowly optimized fast ship would lack capabilities and protection needed for varied missions. Torpedoes still had utility against damaged ships and commerce, while underwater protection became more important as carrier aircraft made torpedo attack possible beyond the surface screen. Early anti-aircraft defenses could not guarantee that every attacker would be stopped. Paper speed gains can conceal a ship's reduced ability to survive the missions for which it was built.