Which medium armors do not incur the speed penalty?
My concern is that the NPC Codex lists NPCs wearing breastplate with no armor penalty as opposed to those wearing other armors. (See the CR 4 Cautious Archer and CR 5 Adventuring Blacksmith for two examples of the discrepancy.)
Also, in the listing on the initial equipment page, breastplate reads as 20 ft as opposed to 30ft (no hindrance). I've read a few forums and it seems that everyone suggests wearing breastplate medium armor (to avoid the penalty, yet it still is listed as incurring the penalty?).
What am I missing here?
Best Answer
A traditional fighter at level 3 gets armor training
While it appears the speed of the NPC Codex's superstitious mercenary—a level 1 fighter—is, in fact, incorrect, any fighter of level 3 or higher that hasn't traded it away via an archetype gains the extraordinary ability armor training that, in addition to other benefits, allows a fighter to "move at his normal speed while wearing medium armor," like a breastplate.
This, at least, explains the cautious archer's speed 30 even though the creature wears a breastplate.
Medium armor typically applies a speed penalty
One of the defining characteristics of medium armor is its speed penalty: "Medium or heavy armor slows the wearer down," says Armor and Shields on Speed. While exceptions exist for some specific or obscure medium armors—armors that are made of mithral, for example, "are one category lighter than normal for purposes of movement and other limitations"—, the fighters from the NPC Codex would say if they were using such armors.