Not even supersonic speeds are required for this to be a big issue. Imagine ... you've managed to get the submarine up to, say 300kph or so, and are cruising along just fine. Now, let's say you need to slow down to return to port or a duty station. Making that transition from supercavitation to 'normal' underwater would be a stone bitch! Water is hard.
It’s funny how this problem never concerned them when they developed supersonic torpedoes for some reason.