IIRC a full set of plate armour weighs about 60lbs. How is that not going to make you less agile (no matter how well made)? And yes, I understand the old trope about knights lumbering around, not being able to get up if they fell over and not being about to mount a horse is all garbage. And that 60lbs is roughly what a modern combat soldier carries. But it is still 60lbs!