Why fast battleships remove Hood's escape margin

Why did Hood's speed stop offering the same protection once fast battleships appeared?

Damage could take away Hood's ability to disengage, while fast battleships could keep up and survive return fire behind battleship armour.

A viewer suggests that Hood's speed protected her until the fast-battleship generation arrived. Against slower battleships she could decline the action, but battle damage could reduce the speed margin she needed to escape. Fast battleships combine enough speed to pursue with armour built to remain in the fight. The transition therefore undermines the battlecruiser's defensive reliance on speed before it necessarily renders the ship mechanically useless.