Geyser's Freeze effect

[BB Shockwave]

I have noticed in KB-TL (not gotten that far with my mage in AP yet to use it) that Geyser has the additional effect on certain creatures that it slows them down (-1 speed) like the Ice Snake or the Archer's Ice Arrow effect. Oddly, this is not mentioned in the spell description.

The creatures this spell affects (ones I noticed so far) are:

- All kinds of Demons
- Green Dragons
- Red Dragons

So naturally I assumed the spell affects fire-based creatures this way, however it did not seem to slow down Fire Dragonflies or Fire Spiders. Anyone knows the exact way this works?