In this video from portrait painter David Cobley
the camera has been set up so that you see the artist and his reflection in a mirror, enabling you to see his eyes moving from paper to subject, subject to paper. (The figure is standing behind him; he's seeing it in the mirror.) Skip the first 50 seconds or so, which is them setting up the scenario, but then watch his eyes and movement of his pencil.
Notice how he's looking at the subject as much, if not more, than the paper. How sometimes his eyes have already moved back to the subject while his hand is still moving, and at others he pauses the pencil while he takes another look.
Note also that he says he's been drawing all afternoon, that he's working for fluidly for it. Practice makes a difference even when you're very experienced!