Usually fuel.
You would almost always go at the most fuel efficient speed (~7knts) if you are not chasing something or running away from something. This is still the case, even if you have 80% fuel, but no torps. You Still go home at cruising speed.
A) Fuel is expensive.
B) The motors will have a longer life.
Exceptions where made in dangerous waters (Biscay post '42).
__________________
|