I should hope that your Pherosoma would outspeed a Dugtrio, it's one of the fastest Pokémon in the game. In-game, however, you'll rarely come across a Pokémon faster than Dugtrio if you only pit him against Pokémon he's strong against. There are few Rock, Fairy or Electric types in the story that outspeed Dugtrio, plus you have the advantage of being a few levels higher than your opponents. Even those that can outspeed it, like Kukui's Lycanroc and Tapu Koko, will have difficulty KOing Dugtrio with resisted attacks.
