You need to qualify your question a bit. Some sails are pretty minimal and downwind only. Some are more elaborate and may require a mast and an outrigger or another means to stabilize the boat and reduce side slip. These make your boat more like a conventional sailboat. Many require that your boat have a rudder of some sort.
Beyond that, it obviously all depends on the wind speed and direction. A downwind sail is just that. A more conventional sail cannot go directly into the wind and is generally not very fast going directly downwind.
Whatever the sail, if used in optimal conditions for its design, it will often be just as fast as the same boat without a sail. Keep in mind that most kayakers cannot maintain maximal hull speed indefinitely.