" When this item is held, the user has an 18.75% chance of attacking first if the moves all Pokémon use are within the same Speed Priority level. " ~ Description of Q-claw
So in the situation that you're using a move with 0 prioity ( normally move )
Mach Punch would go first. In the case that you have another move with +1 priority, such as Aqua Jet. There's a 18.75% chance of you going first.