He knows this by the way the girl stumbles, and it occurs to him for the first time that by taking her with him he'll be slowed down. He'll be hampered by her ability to see.
The character realizes that the girl’s clumsiness will hinder his progress, suggesting a newfound awareness of the challenges that come with her presence. He understands that her visibility limits his freedom of movement, causing him to reconsider the implications of bringing her along.
This moment highlights a conflict between the desire for companionship and the practical setbacks it entails. The awareness of how another's perception can affect one's own journey underscores...