They travel in the range from 2 - 40 kilometers per sec depending on how close to the Sun they get. When they are very far away from the Sun, their speeds can be as low as a few kilometers per second. Close to the Sun, they can get well into the double-digits. At about 40-70 km/sec they are actually 'interstellar' because the Sun's escape speed is of this size, and the comets must be in hyperbolic orbits.