It seems there's a solution in the Bioware forums. "It's client side loading of 13+mb of horse animations. Since the NWN client by default only caches like 16mb in resources, so whenever a horse appears, it causes a momentary stutter as the client has to clear out the resource cache, then load the animation data off the HD. The standard humanoid animations cause no hiccup because you're almost always guaranteed to have it in memory being a PC so it's always loaded.