How does popping up a new set of elements but merely on the same page entirely remove the "visual processing" step? You still have to look and work out how the form works, much the same as if it were on a new page.
Eyes tend to automatically fixate on motion. When the whole page transitions the eye has no single point of change/motion to fix on and must scan the whole page to identify the shapes that represent a login form. With a dropdown, the only change/movement is the login form, which the eye is naturally drawn to.