You're entitled to your opinion, but other forum users have calculated that 27kn would be a reasonable balance at which fuel burn is not significantly higher.
This one is fact, not opinion. The Stena Superfast X is capable of a speed of 27 knots. To drive her that fast through the water, she has 4x 11500kw main engines. She can carry 1924 lane metres of freight.
Her replacement on the Holyhead - Dublin route, Stena Estrid can take 3100 lane metres of freight, but only needs 2x 12500kw main engines to drive her at a more sensible 22 knots. That's 3 times the fuel consumption per unit load for an extra 5 knots of speed! Yes, the two ferries are a generation apart, which perhaps makes 10-20% improvement in consumption, but even so, the power needed to take a ship towards her hull speed is just scary!
Incidentally, both vessels are a similar length, but Superfast needs a much narrower and finer hull to achieve this speed, which is why her capacity is much lower. This also impacts her deadweight (maximum weight of cargo that can be carried) - she can take 5700 tonnes, compared to 9777 tonnes on Estrid. Estrid also has a much larger accommodation block, and a dedicated deck for 120 cars on top of her freight capacity, so she is a bigger, better ferry in every way.
Remember that a faster ferry only saves journey time when at full speed at sea - their manoeuvring times would be the same, if anything a little higher as their long thin hull forms don't go sideways or turn quite so easily. This element is normally around 1 hour of the voyage, so is particularly relevant to shorter crossings, such as those to Dublin. On a 98 NM crossing the saving would be under an hour, from just over 5 hours to just over 4.
What is a more of matter of opinion is whether tripling the fuel cost for a 20% reduction in journey time is commercially viable. In some cases it could be, but they are very few and far between. A lot of companies have switched away from Superfast type ferries, or are running them at more conventional speeds - certainly Stena Superfast VIII, IX and X always run at normal speeds, mainly on two engines, making them far more efficient than they were when running around at top speed.
According to current Google Maps estimations, London to Fishguard is 4hr48 mins by car, and London to Holyhead is 5hr22 by car. Even by adjusting for differential HGV/car speed, it's still faster.
It's not just about the South East though. That's one source of traffic, but those in South Wales/the West Country form a sizeable bloc of demand too, and cross-Wales roads are abysmal.
There's precedent for extra South Wales to Ireland routes being specially commissioned because of demand; look at the temporary resurrection of the Swansea-Cork route, as an example.
Services from South Wales are dying slowly. Both operators to Rosslare have recently downgraded their ships, and Swansea-Cork limped along and would never survive with today's crew and fuel costs, let alone be able to compete with Ryanair. The drive to Fishguard is slow, tedious and unreliable - personally I would far rather take the extra 30 mins drive, rather than battle the Bryn Glas tunnels and the long trek from the end of the M4. The port facilities are antiquated and inadequate for most modern ferries. The routes are exposed to the full force of the weather. Personally I think that both Stena and Irish Ferries are playing a game of chicken, both hoping that the other closes their route first, as they really must struggle being in competition on such a marginal route.