IIRC it slows them gradually but continuously, so the farther away the firing ship is the more of an effect it will have. Also, the ship it's on is fairly maneuverable for a capital, so if you try to maneuver either away from the source of fire (effectively giving it more distance to slow the projectiles) or out of the line of fire (because they'll be slower) it's even better. Also, every little bit helps.
Of course, it's at its most helpful against things that are already slow and that you really don't want to get hit by. Plasma cannons and Reaper torpedos come to mind -- I'm pretty sure it works on all projectiles, not just ballistics?