The average US military soldier today carries equipment that is heavier than wearing full plate. Guns and ammo, their armor and their massive backpacks full of equipment, enough food and water to last several days, and some personal items, all of that can easily add up to 70 kg. And they are still expected to be as agile, mobile to avoid getting shot to death. If they ran as fast as the "heavy" infantry did in that Purzelblume video they would either be shot dead or deemed unfit to join the military and be discharged from duty.
Also, people today are just simply straight up weaker than people of medieval times. Conscripted recruits in europe are usually made of farmers, who spend almost their entire lives working the farm every single day plowing fields, carrying food and water, building the houses, and much more. These are physically draining tasks that build up muscles. Knights or nobility spend their entire childhood training how to ride horses, use various weapons, and very likely training their strength up until the age of 16 or so. The myth that "heavy" armor is super slow and heavy needs to stop.