The effect don't really take effect on a instant, she gonna get speedblitz before.
Like here, she revelead herself to use the Madness Manipulation on them, and they have a moment to react to her prescense, and then they go crazy.
Also, It makes sense in how supposely It works.
The Miasma is like poised mana on the air, and people absorbs that mana trought they gate, which in turn take effect on them. So, not really in a instant.