Capital-ship classification depended partly on the speed of the contemporary cruiser line. When Invincible entered service, roughly 25 knots was enough to stay with contemporary cruisers, but the expected cruiser line had risen toward 28 knots by the First World War. A battlecruiser or fast battleship that could not maintain contact with that screen no longer fulfilled the role implied by the label. This criterion explains why Queen Elizabeth's planned speed might have qualified under earlier assumptions but failed once cruiser requirements increased. The doctrine-based test complements comparisons of guns, armour, and maximum trial speed.
Notable Point · 1:12:42-1:14:00
Why the 28-knot cruiser line matters
Battlecruisers and fast battleships had to keep contact with the contemporary cruiser line, so speed mattered in relation to the ships they screened.