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If the holder's current species is Ditto, Speed is doubled.. The boost is lost when Ditto Transforms.
Although Ditto definitely could use the extra Speed to help itself Transform into the opposing Pokemon, Quick Powder is generally not a good item for it to hold. The problem is, Ditto loses the Speed boost from the Quick Powder upon Transforming, thus making it a useless item for Ditto after its first turn in battle, and leaving Ditto in a worse situation than whatever it used Transform on.
As such, if you insist on using Ditto, it will usually be a better idea to give it an item that will still be useful after it uses Transform. Leftovers fits this well, since it's rarely a bad item choice no matter what Ditto becomes. A Focus Sash could also be useful to keep Ditto alive long enough to use Transform.