User blog comment:Hayashi H/Advanced Equipment Guide/@comment-114.125.11.58-20151024211807/@comment-26486243-20151025071323

Officially, when slow ships are added to fast ships, the evasion of the fast ships will be reduced. However, at present this evasion-nerf is small, that it's not very noticeable against most targets. Originally, adding a turbine to a boiler was supposed to convert a slow ship to a fast one, and the evasion-effect of speed was planned to be much larger, but this was changed before the final release such that now, just using 2 boilers is better for the higher base stat, speed is less significant than originally planned, and slow ships no longer have a way of increasing their speed.

When speed is involved in branch rules (like 5-5, where full-fast fleets will use the optimal route even if the exact combination of 2CV 1CVL is not present), its importance for branching is far more significant.

The evasion-effect is small and not accurately quantified due to the inherent difficulty of measuring evasion effects (the accuracy of the abyssal firing, the fuel level of the fleet, the base evasion of the ship being targeted, the evasion-nerf on fast ships by the presence of slow ones, a possible soft-cap effect on the effects of the Evasion stat and the inherent variance of RNG mean the inherent variance is so high than determining the magnitude of any one of the individual components is tricky). So if you need the higher firepower of a Nagato-class or heavier to increase your clear rate 'even in head-on engagement forms/even in chuuha state' (because any BB can already max FP in parallel at shouha or greater HP), this takes precedence over caring about whether the ship is fast or slow. That is the value of the slower battleships, and for that value, you pay greater resource costs (and reduce the evasion of your fast ships in that fleet a bit).

However, in the event you fight an installation boss specifically, since half of the fast BBs are capable of doing maxed (300x2) damage to installations in chuuha, and all of them completely break the limit when at full health, there is literally nothing to gain from using a slow battleship, but the undetermined speed-evasion malus applies. For that reason sortieing a slow battleship against an installation boss is never a good idea.