I was told that you are good to ask a question about Impostor (Among Us). Instead of average human and possibly subsonic combat speed, why is it not average human movement and subsonic combat speed? Definite subsonic movement speed is even debatable.
When an Impostor kills a victim, they zoom forward in a short burst, which is combat speed. When in a ventilation system, the player presses a button to immediately get to a location instead of controlling movement regularly, so it is also a short burst that is combat speed. The reason Impostors are debatably subsonic with their movement is because the only time they use their subsonic speed is when they are doing something that would confirm them as an Impostor. As an Impostor, the challenge is to kill all Crewmates while blending in with them, so going at a supernatural speed would draw the whole crew's attention. That is why if a Crewmate catches an Impostor come out of a vent, the Impostor can only get to the emergency button far faster than the Crewmate by going back into the ventilation system. If they went at that speed to get to the emergency button through the hallways, they would be easily distinguishable from the rest of the crew.