Short answer - yesLong answer - yesTypically mounts speed vary from 25-60% in most MMORPGs, I'd imagine AoC will be the same, just you can apparently be mounted so you can potentially increase the speed of them yourself. You know. Like pokemon!
A side key part of this is that the skins from the cosmetic shop are just that skins. They go over a mount that you obtain in game. So if you want your mount to have a certain ability you have to get that ability on the mount first, then put your chosen skin over it.
Do we have a specific quote on speed? Not that I can put my hands on.