That's why you have a riding horse and a fighting horse and a pack horse, and your servants have horses and pack horses of their own. When the author of the book talks about "considerable numbers', that is what is had in mind. And it's not like marching infantry doesn't get tired.
Slow cavalry may make theoretical sense but history completely does not bear that out. Completely. And this is a historical game.
A note: the way I read it, your argument seems to be headed the wrong way: The number of horses, the supplies needed to feed them, the men needed to take care of them mitigates
against strategic speed, not support it.
Granted that cavalry is inherently faster. But show me a list of Western European battles that are PURE cavalry formations. And if they are, then they are localized, that is, relatively tactical engagments involving charges, or detached forces away from the main elements of the battle, or even raids against civilian forces.
However, a short listing of important battles on the field (in the early time frame of EU3) indicate the contribution of infantry to some extent: Nicopolis 1396; Tannenburg 1410; Agincourt 1415; Vitkov 1429; Constantinople (siege) 1453; Brunkeberg 1471.
The reason I mention these is because important battles were ususally mixed forces, and that pure cavalry forces in battles was RARE. Feudal European armies were mainly mixed units.
In general, the siege was more important than the battlefield during this period, and cavalry is less useful in that type of operation. I will concede that small tactical operations can be exclusively cavalry. However, these don't win wars: sieges do; occupation of provinces do.
So, according to scale, cavalry skirmishes would usually be
sub-provincial movements, for the most part.
I believe the strategic impact of cavalry is also sub-provincial; that is, it is handled by the attributes within the battle itself. Cavalry pursuit would occur within a province and not out of it: It would occur near the battlespace. And, it would be pure pursuit: there is no valid reason to have a cavalry force move into a HUGE province and find the enemy if that cavalry force has broken contact with its foe. Rather, it would remain in contact and harass the enemy routing formation. With cavalry having superior speed in game terms, this pursuit will not take place, creating the anomaly of having cavalry
waiting for the infantry it has just defeated.
Also, since EU3 is a grand strategic game, the scale needs to be taken into consideration. This is a question of symbolic forces represented as predominately of one kind or another. We can argue all day about "panzer" cavalry and "stick-in-the-mud" infantry, but the formations are not pure, just
representative.
So what if small level skirmishes with cavalry are faster? In the scale of the game, it's not as relevant. At the level we're playing at, the "pure cavalry" army is false: it may be
predominately cavalry, but not exclusively. The designers even say that artillery regiments don't represent 1000 guns, but all of the troops necessary to service the guns. The actual gun count is smaller. So, if you grant a regiment of 1K horse, how many extra men and material is off on the sidelines slowing them down?
Napoleonic Warfare of course had the cavalry charge, but again, this is in conjunction with Infantry, and is usually tactical. Therefore the speed advantage is moot. I admit to not being up to speed on Napoleonic era battles, it's been many years since I've studied the period, so I can stand to be corrected if I'm wrong.
So with that being the case, I still stand by my point. Tactical speed is not the same as strategic speed, and slow cavalry is as much realistic as "fast" cavalry.
Now given all that defense of my position, I still think that Asian cavalry forces are a completely separate case, and need special handling. In the Mongol hordes, they depended less upon baggage trains, foraging off the land, which increased their rate of march. So, it might be correct historically to increase asian nomadic cavalry speeds, but again, not so much for the Western counterpart.