OP, I earlier discussed rails but if your speed is truly at a premium, the most powerful c1 weapon in the game after the rail gun is the Enforcer Cannon.
A lightweight Enforcer is a superb general purpose weapon with (close to) zero mass, albeit as with all fixed kinetic ToT weapons, would require a lot of concentration.
On a speed build I would choose a c1 Enforcer over a c2 Multi every time.