This might seem useless but it can actually help u a lot. For example running from enemys or trying to get to a artillery piece before somebody else gets it.
This is specially for the NW DLC

Cavalry and generals: Athletics: 0

Everybody else : Athletics: 3
BUT Artytillery train guy has a athletics skill level of 3, although he is mounted (!)

-> without any equipment every class except the mounted ones are the fastest

The higher the weight of your equiped weapons, the slower u are.

These are only the most important weapons, go here to see the weight of other http://www.fsegames.eu/forum/index.php?topic=1108.0

Armour-weight
(dis information may not be 100% correct, i got this info from some weird forum page)

1. Every class has the same armour weight EXCEPT:
- sailors (less weight, because they have no chest armour)
- heavy cav (more weight)
- light inf (less weight)

If u run behind somebody slightly left or right of him , u get a speed bonus of 10%. (might be 20%, i don't remember exactly)

thanks for reading, i hope i helped u a bit :33