Denuvo hasn't caused any slowdowns, at least none that was very noticeable, just like it hasn't destroyed HDDs. It does have a slight ressource hit obviously and completely fucks with some types of modding, but it really doesn't do a whole lot other than that. Considering how many games use Denuvo, if it caused heavy slowdowns it'd be a pretty big problem.
Now I'd still rather they don't use it, or at least remove it after a couple of weeks if they're worried about initial sales(which is really where most of the piracy losses are) but there's a lot of Boogeyman stories about Denuvo that are mostly random unsubstantiated bullshit. They might also have Denuvo because they're tired of people fiddling with FF and modding them with shit, Square seems really weird about the whole modding thing, asking people not to mod licensed content or 18+ content, as if somehow it could impact them directly.