Comets are usually in highly elliptical or hyperbolic orbits.
The speed of the comet depends on its orbit and its position with respect to the Sun, which is the focus. For Comet Halley, the speed is 54 km per second at perihelion and 0.9 km per second at aphelion. For Comet Enche, the speed is 70 km per second at perihelion and 6 km per second at aphelion.