The general idea how to EV train is the following:
- Give the Pokemon you want to train a power item
- Have a strong Pokemon with an all-opponents attack (i.e. Heat Wave for Charizard, or something like Earthquake or Surf) on the first spot
- Have a Pokemon with Sweet Scent in your party.
- Enable Exp. Share
- Go to a location with the hordes (find those locations from this answer)
- Use Sweet Scent, kill the horde with your Strong Pokemon in one hit
- By the Exp. Share your targeted EV-training Pokemon gets experience, and therefor EVs. The power items and Pokerus affects the amount of EVs gained.
So your EV trained Pokemon is never in battle. Therefor the reduced stat is not a problem at all. The EVs of speed are not reduced by using a power item, which probably is the source of your confusion.
I use the above method to EV train my Pokemon, and it is done in less then half an hour, probably less.