if your total effective armor weight is >10 you begin to see decreases in wpf.
To calculate effective armor weight the formula is:
(4*glove weight)+(2*helmet weight)+Body armor+Leg armor.
Equation to find your actual proficiency is:
Armor weight modified proficiency = base proficiency * (1 - 0.01 * effective armor weight-10)
Sample calculation:
I wear Black Lamellar (9.1 weight), LSG (0.3 weight), Khergit Leather boots (0.5 weight) and no head armor (0 weight) and have a base xbow proficiency of 171.
so my effective armor weight is...
9.1+0.3*4+0.5+0*2=9.1+1.2+0.5=10.8.
I am 0.8 over the baseline where I start losing wpf so my modified wpf is 171*(1-0.01*0.8)=169.6 effective wpf.
other points:
-Low weight allows archers to run faster
-The upkeep for bodkin arrows (most common archer ammunition) is already high. To balance their expenses archers typically take light armor.
-If you're being chased, drop the bow and arrows and enter into melee (or kite more, the choice is yours).
-Weapon weight only affects running speed, not weapon proficiency