Is there any change in ASPD in the window? What does it change from, and what does it change to?
No.
Is there any change in observed attack speed?
I don't see a noticeable difference, but that isn't saying much because of several factors.
Is it supposed to make normal attacks holy,
Yes, a 12% chance of performing holy element damage. At least, that's what I gathered from iRO's and kRO's skill description. The holy element isn't being applied as you would expect, but this may very well be intentional.
or just stahl horn by a chance? Is stahl horn always holy with it up?
I believe a holy element component is always applied to Stahl Horn if Goldene Ferse is active, but the skill still remains mostly non-elemental. You'll do less damage if you use Stahl Horn on holy element monsters when Goldene Ferse is active.