Not necessarily. Fast(er) motion is often created by skipping pixels and letting your brain fill in the blanks (kinda like what’s happening with the scanlines!). That is, instead of a pixel’s location over time being 0, 1, 2, 3, …, it’s 0, 2, 4, 6 … . That will still look like smooth motion, just with the latter looking 2x as fast as the former.
Also, as dave_j said, you’d have scanlines moving at different rates for things like parallax scrolling, etc.