Magick is the human method of harnessing the energy inherent in the universe. Even in the most extreme circumstances, it cannot circumvent the laws of the natural world. Thermodynamics is the law that most affects Witches, and effects all spells. Matter and energy cannot be created or destroyed on any level by magick.
That said, thermodynamics also affects spell speed, as it were. The fact that all excess energy must be converted into entropy (heat) is a consideration that all must take in casting. However, magick must first accomplish this task. Needless to say, in spells with a greater than average power drain, a large amount of entropy must first be created before the remainder can have its affect. This explanation also serves to tell why large spells, such as those created by special effects teams in movies, are unrealistic.