PokéBase - Pokémon Q&A
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2 Answers

4 votes

No it isn't permanent. It's only cut in half when you hold the item. On the plus side you get more Attack EVs.

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. ~ Bulbapedia

Source: Experience

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2 votes

No. only for as long as the Pokemon holds it. When you remove the item its speed will be normal.

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