Slower than anything else, I can say, even though there are no walls and all varieties of Charm Peddlers. :)
Actually Ive found invasion to be remarkably fats in limited (compared to masques). In masques sealed, several decks can take an hour to drag out a game. Because of all the protection going around, I've found invasion games end much more quickly (this doesnt mean it's a quick set, it just doesnt stall all that much that I've seen).
I noticed the lack of Walls, too and things that lock up the ground war. Because of this, I underestimated the power of green fatties when I played!