Fusion reactions release energy on the order of 1% mass conversion, that is, using E = mc**2, 1 gram of hydrogen fused into helium releases approx (0.01) x 0.001kg x (3 x 10**8 (m/s))**2 = 9 x 10**11 J.

Assuming complete theoretical propulsion efficiency, (all available fuel energy is converted to impulse for the rocket) this engine would have an exhaust velocity of about 1% the speed of light. This fusion rocket attached to an interstellar spacecraft massing a 1000 metric tons (about twice the max takeoff weight of a fully loaded Boeing 747) would push this spacecraft up to 1.342 km/sec with the first gram of fuel.

Optimizing all the engineering, and using the most optimistic assumptions, we wind up with a spacecraft which is 80% fuel by mass at takeoff and its final speed at fuel burnout would be 4800 km/sec, 0.016c, less than 2% the speed of light.