"Moving around should be slow"
I don't see any gameplay justification for this. Movement is a resource and it's use in exploration, commerce, and warfare should support making those mechanisms interesting and fun. "Constant slowness" either could or couldn't achieve this, outside of knowing anything else about the game. Nothing is a go-to answer.
Even "Having to manually move units all the time," which is the current maxim before airlifts, is not a go-to answer. I know a lot of people like the movement rules, and cite previous iterations of the game as good examples. Look, it was fine for a PC game in the 90s to act like a table-top game with bigger math problems. It's fine for a game to act like that now, too, but it doesn't mean that that is what this specific game, Civilization, should be doing anymore.
Civ should first ask itself what it is and then how all game mechanisms can make it the best, most fun, version of that
The V rules were never the best answer. Constantly ending turns on hills for free defense was gamey (though it didn't really turn your 2mp units into 3mp, unless their ideal path to a destination alternated hill and flat every turn; probably out of 10 turns they might have progressed 2 extra mp along the ideal path, with a lot of zig zagging for hills on the way). But more to the point:
-it took too long to get to the front - it was nice to learn what armies were capable of and plan your entire game build order on getting an invasion force to the enemy by turn x, and then steam rolling them, but it was this same rigidity that was complicit in unit imbalances and by extension tech tree imbalances that made the game rote. A good system would let you start a war, realize you are in trouble, and then make it possible but not guaranteed that you could send reinforcements in time.
-it took a pretty perfect amount of time to move units from one end of your empire to another for defense. You could lose a city in that time. You could see another army show up at a different border. It was exciting. The possible and not guaranteed. It didn't give you a free pass for not anticipating an invasion, but it still punished you. So: The road movement speed was good and should not have been changed.
-exploration could have been a little slower early game. Typically with two scouts you saw way more than your empire's footprint, so you were seeing more than you needed to. But by mid-game, the scaling of footprint to exploration flipped: you typically set a target of having the second continent mapped out by 200-250 and didn't worry about expanding your empire before then, because the rewards were too small. Deprioritzation of exploration contributed to the mid-game being a race to industrial, and hurt unit balance some more (e.g. who cared about trebs and lancers).
-Commerce was always an ugly step-child to growth, but the movement rules didn't fail to make it a little more interesting.
VI did not ask itself "how should movement work to make the game systems fun." It said "what would a table top game do," and left so many possibilities for growth as a game untouched.
In my previous post I acknowledged that on gameplay terms some people like the new system and some people don't, and gave it a really failing grade for realism.
Now I want to point out that the movement system in VI fails at unifying the map and the units that move about it, and that is why warfare and mid-game exploration are too tedious.
How are units unified with the map? Well, they move on it. They get to different spots of it to do different things. But what "moving" means depends on what they are doing.
When you start a battle with many units, they are no longer travelers marching between cities. An English bowman from V is not shooting an arrow across the length of London three times. The map has expanded. Or, "The representative reality of the map has zoomed in." Hills and rivers are mounds and streams. Horses flank around from the back of the ranks, not one city over.
When units travel between cities, the representative reality of the map has zoomed out again. The map has contracted.
The problem of V and VI as "a video game" is that they give too few ways to speed up that (really very boring) part. That part does not serve the game at all (the unit is not exploring, or warring, or engaging in commerce: we should not be wasting clicks on it).
Time expands next to a unit and contracts away from it as well. What is happening to a unit 3-5 tiles around it is "now." But the parts of the map more than five tiles around it is "the distant future." Techs and civics will have evolved by the time it reaches those tiles. It will be obsolete.
This is how the map separates the unit from the rest of the game. The map divorces the unit from your empire's growth, and this contributes to non-ancient and non-modern eras in V and VI never feel like they are really happening. Slow movement does not enrich the game. Movement rules need to let the unit transcend the map when getting to areas more than 5 tiles away.
Yes, exploration should be slow. Even a bit slower than V. The pace at the very start of VI is not so bad. (By turn 50 I have a hard time continuing to grow my revealed map, and that has a bit to do with movement rules needing to loosen up faster for scouts and a bit to do with the confusing FOW depiction.)
No one ever accused Final Fantasy of being a fast game. But for the most part once you have explored an area enough to be bored with it, you get to teleport through it. Exploration and growth were slow in FF, but that didn't mean "movement should be slow" within the game. It wanted you to re-engage with all the locales on the map that you had spent previous time uncovering. It wanted the map to be yours.
Civ could do the same. Civ could implement unit teleportation for reinforcements and in so doing, make the map richer, make the tech tree richer, make unit balance better. The goal of unit teleportation would be: units could get to spots of the map more than 5 tiles away before the time metaphor breaks for the unit; and: The AI and human do not have to waste thinking on moving through explored areas.
It's really intuitive and obvious as a concept at that point, the only thing left to hash over is the implementation, the rules. Again with the guideline of making warefare etc fun and interesting.
Rules like:
-teleportation consists of dispatching and recalling units from your borders.
-A unit within borders can be dispatched to anywhere in viewed (not FOW) map, and it takes 5 turns to get there (faster after unlocking better techs)
-It might appear up to 2 tiles away from the target tile.
-It might be intercepted by barbs (think RPG teleportation again).
-if you don't keep vision with the tile, the unit reappears in the dispatch city.
-units anywhere outside of borders can be recalled to a city in the same number of turns.
Aside from some other obvious benefits, especially to making the AI a better opponent, I see the value of teleportation as being: Now units are only using "movement" within the zone of the map that is their relative "now." That means all units are existing in the same time as each other and as your empire economy. They do not fall down into a hole of tiles on the map.
Back to units and the map they are on:
-If 2mp units can teleport, then the VI rules are ok because they are purely for the other things the unit does on the map, warring or making improvements etc. Though we still need to let units end on cost 2 tiles with only 1mp, and start the next turn at -1, because the UI issue of ending turns is unsolvable otherwise.
-Scouts still are your explorers. Since teleportation doesn't help reveal new territory, you would never use it on a scout. Exploration pace is not affected by teleportation, except if you open a new landmass and bring a second scout to it.
-Horses might still run about to patrol. In battle and patrolling, their higher mp has the same positive value compared to 2mp units, which is elegant. But 2mp units aren't good at purely moving on the map and shouldn't ever be used for it.