Even twice that speed (16 knots) is hardly a sprint. Sailboats of yore could probably better 16 knots.
Most modern cargo ships cruise at around 15 knots around most economical speed, a trade off between fuel consumption and number of trips you can made. This is a function of fuel price. Before 1970, ships traveled at 22 knots + , after fuel crisis , speeds dropped to 15 knots. Today, it is only "high speed" ferries and some container ships that go to 22knots and beyond
It strikes me if I pull out high school physics from the recesses of my mind that boats can be speeded up if you reduce their area of contact with water by cheating - as in hydrofoil boats, or by sheer raw power. I suspect (with no proof) that at speeds below 30 knots a ship that weighs 5000 tons and a sub that weighs 5000 tons would have a roughly similar surface area in contact with water. So if the ship can do 25 kts on the surface it should be able to do the same underwater given the same amount of power.
Some basic YinJin Ear Ring here. There are two components to a ship's resistance. They are 1) The wave making resistance, you make waves, you lose energy. Waves are made because of the air water boundary on the surface.. this is measured by the Froude Number (google for it) and 2) The skin friction resistance, which you alluded to, that is a function of the Reynold's Number (google for it).
Coming to ships and submarines. Now unlike a surface ship , a submarine has NO
, repeat NO, Zilch, Nada, Zero, Cipher, Shoonya wave making resistance, because it is fully submerged and there is no air/water boundary to make waves.
So, your point that a ship with a given wetted surface area is going at 25 knots, a sub with the exact wetted surface area has to go at the same speed is not true. The sub in fact will be going substantially
faster (assuming that both the ship and sub are hydro dynamically efficient and "ship shape" and have similar powering).
Google for the United States "Guppy" program and why it came about and why modern submarines have a "tear drop " hull shape, compared to WW-II subs which have a shape like a "surface boat", with the top sides fully closed.
So I wonder if low underwater speeds have everything to do with stealth. Apart from engine/gear noise - the ship will surely set off turbulence that is detectable. Also I am guessing that no matter how high tech and stealthy you make the propeller/screw the faster you turn it the more likely it is to be noisy.
There is a tactical speed beyond which your sensors will become "blind" with your own noise (though modern signal processing probably obviates some of this.. think Bose active noise cancelling headsets) and passive sensors will get lost in all the noise and you might have to start using active sonar . And yes, true about the propeller/screw, but with good and careful design, you can make sure that it is well manageable within the design speeds.
That means that stealthy subs cannot turn on full power in one go and "burn rubber" as it were by letting the screw go whirrrrrrrrrr "Avast! Full speed ahead!!" as the ship accelerates. Even the acceleration must be slow to avoid turbulence and noise. That means the propeller is gradually accelerated from whoooooosh-whoooooosh to whooosh-whooosh-whooosh to whoosh-whoosh-whoosh-whoosh.
There is such great inertia in the system and ship machinery, that in any case, it is not a "quantized jump" from say 200 rpm to 500 rpm like in a motor boat with an outboard engine in a juffy. More than the prop, usually in ships/subs much of the source of vibration is not the props per se, but other rotating machinery, in fact the drive shafts (long rods, prone to vibration) are notorious, and hence efforts to isolate all machinery on mountings so that they cant transmit vibrations and other stuff. Subs with the motor in the aft and small short shafts and single screw will have mercifully less of such a problem. Put the engine amidships and long drive shafts driving props in the rear like most surface combatants and older WWII types...yeah, big problem.
In fact, even flushing to toilet in a submarine has to be extremely quiet. If you take the "parryware" toilet or even the TFTA "American Standard" or Uber TFTA Japanese "TOTO" and install it on a sub and if a sailor takes a dump and flushes, the "woosh" can be picked up if not isolated.