The person drawing slowly may be noticing distinctions that quick looking collapses.

Looking fast is a kind of summarising. The eye takes what it needs to identify the thing and then moves on, so a hand becomes a hand rather than those particular knuckles and that particular light. Drawing forces the pace down until the differences reappear, and what looked uniform turns out to be full of decisions.

You do not need to draw to use this. Sit with one object or one view for ten minutes without doing anything to it. Whatever you notice in the eighth minute is the sort of thing that ordinarily gets summarised away, and it is usually the interesting part.