Fiendfyre would zap the magical energy stores of anyone who tried to sustain it. That was a lesson hard-learned by Hermione during the months she trained to cast it with confidence with Felix. Considering she hadn't cast the spell in a long time, years possibly, she was fortunate that it worked at all. Conditioning was just as important in magic as it was in physical exercise. If she wanted to cause more devastation in the future with the same powerful energy, she needed to keep practicing. Energy didn't just magically appear out of nowhere. It had to be taken from somewhere. Endurance was key.