The speed of decal animation seems to be hard coded somehow - though I dont know that for sure - perhaps someone else can comment. Presumably its linked to the number of images to view on your decal image and "speed" of display which we have no control over it seems. Its fairly complicated and an overview from someone from TGC as to what exactly controls the speed would be helpful. e.g. is the speed relative to the number of images having to display within a specific time period and if so how can we affect or control that? It seems we cant?
Still looking for a workaround to force some kind of speed control - looking at image numbers, size and so on, without much success at the moment I am afraid - I will let you know if I find a method.