The main cause of her increase in speed was her Noble Phantasm—the Maiden's Chastity, Bridal Chest.

It is not simply a weapon for bludgeoning the enemy; if anything, that is a secondary use. The true ability of her Noble Phantasm is to absorb prana. In any Holy Grail War—ruthless struggles between Servants and magi—converted prana will end up dispersed all over the place, eventually dissipating into the atmosphere.

Her Noble Phantasm, the 'heart' of Frankenstein's monster, holds the ability to absorb excess prana. This amassed energy can then flow through Berserker's heart and into her Magic Circuits, even allowing her to emulate a [Prana Burst]. Of course, it is not an all-powerful weapon—but having taken form as the Servant Berserker, one would have been hard-pressed to find a more convenient weapon. For a class known for sapping prana reserves dry in short order when fully active, this Noble Phantasm allows her to fight on like a perpetual motion engine.

- Vol 1 Chapter 4