The best way to get Pikachu's speed raised (if Carbos won't work) is battling some wild Pokémons whose EV Yield is Speed such as Zubat and Golbat, Buizel and Floatzel, Pachirisu, Aipom, Buneary, Ponyta etc. This would certainly help you to raise its Speed. You can also use Power Anklet to gain Speed EVs whenever you gain experience, but holding it halves your speed in a battle. If you don't want these difficulties, go on and catch another Pikachu who doesn't have -SpA and -Spe Nature.