This may be silly but I started wondering about this after looking at this question: How does the Armored Kilt work?.
I have not been able to find an explicit rule that says all Medium armors reduce speed in a specific way (and similarly nothing on Heavy armors). Sure, armor lists show all medium armors slowing 30ft speed characters to 20ft and heavy armors doing the same plus reducing run speed... and yet, nowhere is it said explicitely.
My current thinking is that this may be a case of "too obvious to even mention it", or a case of "was mentioned in 3.5 and omitted, but still implied".
Also note that I'm aware this is barely a problem in general since almost every armor follows the same pattern and things that change an armor category tend to be specific enough about how it affects speed to avoid questions. This is more of a curiosity to me.
Can anyone confirm or disprove that there is a strict cause-consequence relation from armor category to speed reduction?
Here are some hints I have found:
- In Equipment, we find the following statement:
Speed: Medium or heavy armor slows the wearer down. The number on Table: Armor and Shields is the character's speed while wearing the armor.
So it slows the wearer down. That's pretty final. There's just one thing that bothers me: it doesn't say how much nor does it give any details. It merely refers to the table. To me, this doesn't say "all Medium armors work that way and all Heavy armors work that other way", it rather says "each armor is unique and only a specific entry will tell you how much it slows you down".
The Armor tables always list the speed reduction (or lack thereof in the case of Light Armors or shields). Why list it if there's such an easy default rule for it? Convenience? Is it possible that some listed Medium Armors would not reduce speed? Or more or less than others?
The Fighter's Armor Training just cancels any armor speed penalty for a given category.
Armor Training (Ex): (...) a fighter can also move at his normal speed while wearing medium armor. At 7th level, a fighter can move at his normal speed while wearing heavy armor.
Admittedly, if armor categories had different effects on speed, this ability may have given a specific feet per round value such as "a fighter ignore 10 feet per round of speed penalty while wearing medium armor".
Barding has a table showing a generalized effect of armor category on speed... but only for animals?
What seems to be the single exception to uniform speed reduction of all Pathfinder: the Stoneplate Armor (Table here which lists a speed reduction of 30 to 15ft instead of the usual 20 (of note: it should also point out it reduces run speed, as other heavy armors). I'm not sure this is much of an argument for either case, but hey, it exists.
Mithral is an interesting case since it modifies the category of an armor but only for certain aspects:
Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor.
Mithral could be a case for how changing categories changes speed reduction, but it has such a specific rule that I think it's hard to generalize. Also, it only applies to "most mithral armors".
- Finally you have the very messy Armored Kilt (Table here) which can be used on its own, as light armor, without speed penalties according to the table (and following the unsaid rule)... or attached to light or medium armor, turning it into a whole new armor a category higher. That's all it says. One could assume it means proficiencies are affected and, possibly, speed reduction? It doesn't really give a hint towards either, sadly.
Note: I know I'm being very lawyerish on this. It's mostly because I like to understand what the RAW is before moving to RAI and house rules. Unless someone can find something saying speed reduction should not be applied uniformly across all armors, I will go with the general consensus anyway.