VV wasn't specific.
EVs are gained on level-up, same as Carbos boosts. That's why the stats don't change right away. You get more points when you level-up then you would've. But the Power Anklet gives you more EVs, but reduces the actual stat. So yes, if you're maximized your Speed EVs and hold it, then it reduces your speed. But it's only to be used to double Speed EVs, then you put in a different item.