Their speed wasn't so much to outrun battleships, but instead to out-range them more.

Try to remember they were designed for different roles.

The Battlecruiser was to deploy battle-ship size guns and fire-power to the other side of the world literally. They gave up internal armour and sectioning for more space for victuals, supplies to be able to patrol further longer without having to make pit stops all the time. Respond to hot spot faster, so to speak.

So in the same vein, they were better sea-lane raiders as well.