I've encountered in several threads how wonderful a spell Solid Fog is. While removing someone from an encounter is a relatively big deal, at first glace, moving out of an area of Solid Fog should take one or two rounds, and you have already wasted this round on casting it.
At best you can put it straight ahead of a creature to create four squares of effect between said creature and yourself. Then, given the "target" doesn't have some escape method, it should be able to move two squares towards you before having to end its turn. On the next turn it moves again, and now there is a single square of effect between the two of you, allowing the target creature to cast a spell on you (or attack you with a ranged attack, which is probably less problematic).
But I've noticed that there seems to be a consensus (or at least more or less of one) that a creature may only move 5 feet a round while in an area of Solid Fog. While I could assume that’s an application of The Move 5 Feet Through Difficult Terrain rule, I actually don't see why it should be the case. Having a move speed of 5 feet allows a creature to move 5 feet as a move action. Also, there are effects, like Entangle and Web spells, which specifically say the target must use a full-round action to move, and Solid Fog is not one of them.
So, am I wrong that a creature caught in an area of Solid Fog may move 5 feet twice? If yes, could you explain why?