Yeah.  They suffer a massive speed decrease.  The Salem Witch tops out at 16.1knts, this from a video I watch of PhlyDaily playing her.  Phantom Fortress will required a skilled CV player to use to her full, while Salem Witch will only need an average player.  Phantom Fortress is an Essex from my understanding, while Salem Witch is a De Moines.  I've already seen some complaints about them, either just sitting there, derping, or owning everything.  The sitting there I see as a valid complaint.  I expect a "Boss" to put up a fight and wrecking people.  I would be disappointed if it were too easy.  It's the other complaint, them owning everything, that I find stupid.  They are meant to be a Boss.  They are suppose to be tough to deal with.
Ah well.  I did note that there is a Halloween Camo you can use that is +100% to XP, but at the cost of 30 gold.  I will likely use it to help push for Yamato faster, but we'll see. 

EDIT: 16.5knts for Salem Witch.