Hermione could spend all day sitting on a sofa levitating a feather without expending much magical energy at all. But, if she tried to cast a series of powerful spells for many hours, she would eventually be too tired and depleted to continue. One could train themselves to undergo a rigorous routine of high-energy magical spells. It took patience and determination and even then they would have to rest at some point to replenish. Some witches and wizards had to work twice as hard as others to be half as good. It was a cruel twist of fate. Blood status didn't always determine a person's magical competency, no matter how many bigoted, misguided Purebloods claimed it so. A Muggle-born might be inherently more powerful than a Pureblood who could trace their magical heritage back a thousand years. These were facts that many Purebloods fought wars to prevent getting out. It was all madness that Hermione had little time or energy for. Maybe in a different world she would've been glad to fight against the pro Pureblood laws, not in the current universe she was forced into living.