The average person was much stronger back then.
No.
The average person back then got sick frequently, had a diet deficient in a lot of basic vitamins and minerals, etc.
Even the average warrior, when compared to a professional athlete today, would look scrawny, underfed, and weak.
If you're in any serious doubts, go look at American Football players from the 1950s, vs. players now. It's like night and day.
Modern nutrition, better understanding of the body, weight training, etc., make the the people who would have been warriors today far more fit than the men of the day. That, and considerably heavier and taller.
In relation to tiring from swinging a sword for hours, knights, if we are to focus on them, trained from the time they were a child. All knights ever did was fight.
That is incorrect. What most knights did was play landlord and work on their small-holdings, so they could afford the gear that made them a knight and keep their fief and social status. Only a smallish percentage of knights would have been wealthy enough to train daily, let alone at the level of a modern athlete.
The average training of a knight of prime fighting age (14-30 or so, back then) would probably be about the equivalent of today's National Guard here in the U.S.- basic training, some training on weekends, a refresher for a week now and then. They weren't like elite soldiers today (SEALS, for example), who train constantly because that
is their sole function, besides making war. The concept that the raison etre of a knight was to make war is largely a misunderstanding of their role in the feudal system. Were they better-trained than commoners? Yes. Were they super-elite warriors? For the most part, no.
The claim of only existing to make war, advanced by the feudal lords (that they existed to protect the common people, and so on), was mainly a bunch of cynical humbug, IRL. The knights were land-owners who explicitly forbade the commoners to keep weapons, kept them in conditions scarcely different from slavery, and generally they weren't "nice people", by modern standards. There is a reason why practically everywhere in Europe revolted against the feudal system the minute that the balance of armed power rested in the hands of the commoners, and it wasn't because the commoners were happy with feudalism.
As for cleaving through horses... in theory, yeah, you can cut through a horse with a sword, like those YouTube videos showing "sword tests" vs. carcasses.
The reality was a bit different. The horse is moving, the horse may be armored, and the horse doesn't want you to kill it. And a frightened warhorse is a very dangerous animal. I seriously think that you haven't ever been near a real horse, tbh- if you had, you would not presume that killing one would be easy.
Cleaving through armor... no. Not even slightly. Knights may have very well chopped off limbs or heads of armored men, striking the weak points with a strong and lucky blow. Although even there, I find that rather hard to believe- even chainmail with padding underneath would have been very resistant to cutting, and most of the tissue damage would be blunt trauma injuries- broken bones.
But the vast majority of kills, IRL, were from penetrating wounds. Stabs, not slashes. It's not romantic, but it's how most people died.