Length of journey times are only an issue if you use a vessel fitted with the cheap engines used on the Irish Sea, there are ferries out there the size of Isle of Innishmore and Stena Superfast that can do Cork - Bristol in 7 hours or Cork - Pembroke in 4 hours.Eh? This makes no sense. Cheap engines?
For the record Stena Superfast X could be capable of 28+ knots if Stena wanted her to. The reason she isn't is fuel burn which increases at a faster rate the faster you go, as with any vessel. The speed of a vessel at a given power output is dictated by many factors, especially hydrodynamics. To use a modern design as an example, E-Flexer is designed to be able to travel at 18 knots on a single engine using a single screw. It takes 2 engines and two screws to attain 24 knots (with the resulting increase in fuel burn). More fuel burned equals higher ticket prices. Even Visentini's have been known to attain 26 knots, but again theres a big fuel burn penalty.
Yes, apologies, I was tired when I wrote that and the point I was trying to make was about cheaper to run engines, and the example was intended to be Stena Adventurer and not any of the Stena Superfasts (which as you say are capable of service speeds of 28 knots+), while the Superfasts are known to be thirsty when operating at higher speeds, ironically they are in the category I was talking about with conventional hulled ferries operating at around 30 knots, while the economics of these higher speed vessels is questionable when competing against slower vessels on short routes (passengers want more speed but are generally reluctant to pay more for it) I have seen some figures where playing with the fuel blend used can significantly lower costs while staying inside emissions regulations, on some vessels engine components and fuel injection systems may need to be changed, if you are a large corporate then this would be no problem, if you are a small operator looking to lease, you might have a job of work convincing the ship owner to make the modifications.
Again apologies for the confusion.