It says in there description they lower speed. But by how much?
The power items are:
Power Anklet: + Speed
Power Band: + SpD
Power Belt: + Def
Power Bracer: + Atk
Power Lans: + SpA
Power Weight: + HP
Macho Brace: doubles EVs gained
>EV enhancing items are items which enhance the EVs gained when a Pokémon is defeated. All of them halve the Speed of the Pokémon holding them in battle while they are being held.
According to Bulbapedia, all of them make you half your original speed.