Sorry DaveW for not addressing that concern. Basically it's a question of when do you check for the slow trait ending the creature's action. I don't think it's made it into the supplement, but the decision is at the end of the move action. That way if something gains slow mid move it still affects them, as opposed to checking when they begin their move action.
Another option would be a constant check, but that doesn't feel right for cases in which it is removed mid move. For example, you start your move action, and your opponent reveals Enfeeble. Well, now you're slowed. You then reveal Cheetah Speed. That cancels it out and you're not slowed. By the time you end your movement, should you have lost your quick action due to being Slowed for just a microsecond? No. That would defeat the entire point of the counterplay.
So, bringing that in to the situation of leaving the Swamp, you lose the slow trait before ending your movement. Thus it doesn't negatively affect you.