[Arelius]

You are actually speaking of the same thing, "high-speed film" similar to a "high-speed lens" doesn't actually affect the framerate, yet rather how fast it can produce an image from a certain light-source. It's simply more sensitive.

Now, you're correct that the actual effect is done by changing the shutter speed. but the loss of light is often compensated for by using a faster film, since using a larger aperture has more significant effect on the scene in the form of DOF.