Having taken off her armor, Saber didn't gain just agility. The prana needed to form and maintain her armor was now available for use in her offensive. This means a lot for the skill of "prana burst" that Saber possessed.

A "prana burst" is the accumulation of prana in her weapon and her entire body, momentarily injecting an arbitrary vector for an exceptional boost of her abilities. So to speak, it is a jet blast of prana for her movements. She only had the thin body of a small girl, but this was the secret that allowed her to brandish her big sword with the combat style of a power fighter.

The surplus of prana can be converted into mobility power for a close-range combat, but when Saber uses up to the prana needed for her armor in a "prana burst" it could grant her six times that bonus in power and speed... This is perfect for the destructive power of a one-hit-kill attack.

The handicap of losing the armor was overturned by the advantage of throwing away her armor. This was Saber's conclusion to deal with the "spear of exorcism" of Lancer.

- Fate/Zero: Act 3, Part 5