The audio description tracks are a different genre than screenreadera perform. They're acting, by actors, carefully written and performed to fit into the gaps in the dialogue while preserving the mood and flow of the show. I think speeding them up or making them robotic would ruin them, while both of those traits are actually desirable for screenreaders.

Ideally that is what AD should be like. too often you set the volume right for a movie so the characters can be heard, then the AD is like an insanely boomy voice that shakes the room. Plus for some reason the also turn the movie audio down, as if that would be necessary.