actual rate of fire in combat is always significantly slower than theoretical ROF during sea trials. sure, you might be able to achieve X rate of fire during calm seas with minimal elevation, fresh gun crews and nothing else to worry about, but in combat your guns are elevated, seas are not likely to be near dead calm, there's all kinds of other shit to worry about, and after about 2 minutes of combat the crew is not going to be fresh. so how would you accurately simulate ROF slow down from the multitude of factors at play? same sort of issue with gun ranges. sure, a real gun could in theory fire halfway around the world. but you'd never hit anything, and by the time a shell got out there it would lose a significant amount of penetrating power to wind resistance. no battles were fought at maximum theoretical range because you're doing nothing but threatening the local fish populations of the next latitude over.
UA:D is after all a game, and game balance is a thing. they're obviously working on the it, but it wouldn't make sense to have a ship fire every round on board within an hour of the opening salvo. at that point just stay at range until they run out and rush in with torpedo.