I have noticed the heavier armor the slower the attack speed becomes. I guess this is because each piece of armor has a speed penalty (under their "speed" stat) which is greater the heavier armor is. Shields have this speed stat as well. How is the overall speed penalty calculated?

This is a tough question. I managed to find this information and formula in the rules I have at hand:
The attack speed is based on the "rest time" between separate attacks. The formula for the "rest time" is following:
DO = RZO + RZA – (X*0,1)

Where "DO" = "rest time" and "X" = mastery level in wielding a specific type of weapon (mastery level of Melee Combat, or Ranged Combat, I guess), but there is no explanation as to what "RZO" or "RZA" means in the document. It seems that "RZO" has something to do with armor (because "RZO" can be reduced by becoming Master in "Armor Use"). Could it be the total Speed (Penalty) value of all equipped armor pieces? And "RZA" is also used when calculating the Stamina consumption during attack...