Fastest game speed is the Fastest optional speed given in Marine Arena. This Speed offers absolutely no base defenses other than the Auto Turret and your Planetary Fortress. Meaning, if you lose your army, you will likely lose since you have no significant defenses other than your army to protect you.
This game speed is recommended for intermediate and expert players, since it revolves around constantly watching your opponents for weak points, in order to take advantage of them and eliminate them early game.
This game speed favors early Zergling builds and heroes.