As Slughorn explained, Hermione knew that "overdosing’ on such a thing had a terrible effect. It couldn’t even be taken too closely in dosage and was highly illegal (and easily proven!) for sports and exams. There were few things Hermione had wanted to experience, but a moment of a pure lucky day seemed extraordinary. To find out that it was a prize for the lesson had Hermione giddy. There were few better at potions than her.