Think of it this way. A destrier could weight around 1,700 lbs. Say a man weighs 170 lbs. He could be carrying 20 lbs of equipment (heavy gambeson, helm, gauntlets, boots, weapon). If he is wearing plate he could be carrying 50lbs of equipment instead.
If you divide this by 10, you can see what it would be like in terms of a person.
It would be the same as a 170lb person carrying a 17 lb backpack, and adding either 2 lbs or 5lbs into the pack. It wouldn't really make a difference. So he is carrying 19lbs vs 22lbs. It might if he was going on a very long multi-day march...but not in terms of a battle.
Add 4 legs into the equation, and it would be even easier to carry.