My tests show the same results. The penalty with shield modifier does not effect weapon attack speed. Using bastard weapon one handed causes a significant attack speed reduction and the reduction is the same for polearms and two handed bastard weapons. The increase in time per attack depends on the weapons speed and can be calculated using the following equations for attacks made with 1 wpf:
Time per attack for using two hands = -0.0124 seconds * (weapon speed) + 2.3368 seconds
Time per attack for using one hand = -0.0195 seconds * (weapon speed) + 3.257 seconds
I have posted on this topic before about
polearms and about
two handed weapons.
I did some testing in the Native tutorial to get accurate damage values (the dummies have no armor and therefore no damage variance) and found that the use of a polearm weapon with one hand yields an ~30% damage reduction compared to using the same weapon with two hands. With a 20 damage quarter staff, one handed damage was 17 and the two handed damage was 24. After modifying the item_kinds1.txt file to make the quarter staff do 100 damage, the one handed damage was 78 and the two handed damage was 109.
Interestingly, the quarter staff does not have the penalty with shield modifier and modifying the item_kinds1.txt to make it have the penalty with shield modifier did not change the damage results. I tried the same thing with a long awlpike in my single player game and removing the penalty with shield modifier did not change the damage the weapon did.
Furthermore, the trigger for the attack speed and damage reduction is not holding a shield but is using the weapon one handed, as one does when using a lance from horseback. So, I really have no idea what flagging an item with penalty with shield actually does or why is named what it is.
EDIT: accidentally posted before I finished typing