How fast are comets?
It is too difficult to tell. There is no existing average for the velocity of comets, as all comets are different. Gravitional, solar and initial velocity have a lot to due with the velocity of a comet.
Even though there is a great deal of variation, the answer would have to be very, very fast. Fast, that is, compared to earth standards. If a comet were to hit earth, for example, chances are that we wouldn't see or even know that it had hit. It would just hit. Not to scare you. The chances of a comet hitting the earth are slim to none.