Since when do subs sail faster than 10 knots on a regular basis, unless their commanders are incompetent boneheads? The slower one sails, the less noise one makes, i.e. the less likely one is to be detected...
Subs sail as slowly (or as fast) as the situation warrants.
And modern submarines are designed to be rather quiet at 15 knots... though 'quiet' is a relative term depending on everything from how deep the boat is, to what type propulsion is used, to which country built the vessel.