Rendering:
The heat haze is just a rectangle mesh rendered into the scene a la imposter (a render is taken of everything 'through' the mesh - this is why its such a frame rate hit).
Animation:
The mesh is actually a grid of nxn cells.
Every frame the verts making up the grid are moved through a noise function. Since the noise function isn't a function of time, then yes, its possible at high frame rates it looks 'too fast'.
So there's no way, iirc, to slow it down
Jason.