Weight penalty = everything your carrying. In my experience leather jerkin is about tops for armour if you don't want to hurt your speed. It is also dependant on your weapon and class. Range carry lots of extra shit so you need to dump kgs in the armour to make it up. Long weapons also effect your speed and sheathing everything helps a lot.
WaltF4 has a guide worth looking, he breaks the whole thing down.
I use 15/24 now, it's lots of fun. It's got enough acceleration and speed to be able to run figure 8s in and around mobs and lol as they smack each other in the face.