It is fun to see the reactions I get when I make changes to CiV through mods. For example, I was working today on changing Knights because at the moment they are a fairly weak unit. There is an upgrade path that goes
Horseman (Strength 11, Speed 4) to Knights (Strength 18, Speed 3)
Strangely, the corresponding melee units look like
Swordsman (Strength 11, Speed 2) to Longswordsman (Strength 18, Speed 2)
You are going to have to trust me on the fact that Horseman and Swordsman units are pretty well balanced against each other. Knights and Longswordsman units should also be balanced against one another... but they aren't, because Knights are only Speed 3. If Knights were Speed 4, things would be fine. I think what happened here is the people who made the game thought along the lines of "Well, Knights have heavy armor, so obviously they are slower than a Horseman. Better make them Speed 3." I encountered this same line of reasoning when I proposed changing Knights to Speed 4 to make them actually usable - people responded that Knights wear heavy armor and so should be slower. Why nobody thought that randomly making Knights worse would ever present any difficulties in strategy is puzzling.
This idea that we can usefully model the amount of speed a unit might sacrifice to wear heavier armor is a pretty silly one. We are talking about a combat model where an archer walks up to within bow distance of a city but does not shoot right away and it is entirely possible for a horseman to ride from another city, along the adjoining roads, up to the archer and stomp the archer into oblivion before the archer can even fire. This Speed rating is a concept used to suggest that mounted units can cover more ground and close distance more rapidly than units that have to walk but it does not significantly model reality any more than that. The other issue of course is that we are playing a game, and if Knights suck because we are trying to make them 'realistic' then we aren't being realistic at all; Knights were powerful and dominant military units for a very long time!
What we need to accomplish is to maintain immersion, not pretend we are modelling reality. Immersion is broken by many things, one of which is gross game imbalance, but there are many others. If Knights were slower than Longswordsmen then we would also find immersion to be broken as there isn't much of a way in which Knights weren't faster than people that had to walk. What we need to find is places where we can reflect real life characteristics of things in game terms that people understand. Knights are on horses, make them faster than people who have to walk. Knights need to have 4 Speed to be good, so give them 4 Speed, which is faster than people who have to walk, so we are all good. Of course you can never please anyone, which is amply demonstrated by the endless forum discussions about how long exactly it should take a modern destroyer to defeat a renaissance caravel and how the game does not accurately represent that conflict.