A mid-14th century knight, wearing mail under his plate armour, would wear 30 kilos of armour, but it's because he wears the said mail under the plates. These plates were quite thin, actually way thinner than the ones used for reenactment (for safety). Here is a 14th century great helm. The helmet weighs 2,6 kg, and thickness of the steel varies between 0,6 and 1,8 mm. It's extremely thin. Cutting through plates must have been rare, but not impossible.