Which navy? Royal Navy sloops tended to be slower than Mediterranean vessels or coast-wise vessels eg USA. Because of the need to be heavier built for deep ocean voyages and conditions in the North Sea and Bay of Biscay.
About 10 knots probably in ideal conditions, but privateers and (especially) slavers could be 3 or 4 knots faster . Again, in ideal conditions, which were,t so common.