There are relatively big varieties, but most of them is between 10 and 70 km/s.
If a comet is a periodic comet, that means it needs to have an elliptic orbit around the Sun. That gives an upper limit to its speed of the escape speed from the solar system on the orbit of the Earth. That is around 40 km/s.
But this 40 km/s is in the reference frame of the Sun. The Earth is moving in this reference frame with around 30 km/s, on a nearly circular orbit.
Between the escape speed and the mean speed of a circular orbit there is always a $sqrt{2}$ relation. It is a physical law.
Theoretically it were possible to find extrasolar comets (if the speed of it were bigger as around 70 km/s, it were a clear signature of its remote origin), but they aren't coming.
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