Smooth scrolling (if turned on) is applied in only for certain situations, to avoid laggy scrolling you have to wait for, which is really annoying and makes people turn off smooth scrolling entirely.
When it happens it's very smooth for me.
Doing a short scroll with the mouse wheel in thumbnails mode is an example where you'd usually get smooth scrolling.
(This thread is also about too many subjects to keep track of. Please Ask one question per thread, and also link your account if you want to request so many changes and discuss so many things.)