I believe it gives around 30% penalty to speed, might be the same with damage.
The penalty with shield modifier is for damage, and a 30% reduction in damage seems to be accurate. All polearms are slower when used with a shield compared to when used two handed, even those without the penalty with shield modifier. The difference in time per attack depends on the weapon speed. The equations for time per attack verses weapon speed with 1 proficiency are:
Time per attack for polearms used with two hands = -0.0124 seconds * (weapon speed) + 2.3368 seconds
Time per attack for polearms used with a shield = -0.0195 seconds * (weapon speed) + 3.257 seconds
Where those equations are coming from can be found in
this thread if you are interested. The percent increase in time per attack for a polearm used with a shield compared to used two handed is:
Speed 104 Staff: 17%
Speed 98 Spear: 20%
Speed 95 War Spear: 21%
Speed 92 Awlpike: 22%
Speed 90 Bamboo Spear: 23%
Speed 75 Heavy Lance: 28%
The percentages remain constant regardless of user proficiency as the reduction in time per attack with increasing proficiency is proportional to the time per attack at 1 proficiency and the reduction in time per attack appears to be constant at ~6.5% per 100 proficiency for all weapon animations.
