Complex sentence: It is not so much the pitch or range of the notes that are played that challenges the musician, but simply the musician's ability to move to them quickly enough; because of this and its complexity, it requires a great deal of skill to perform.
Split into simpler sentences: It is not so much the pitch or range of the notes that are played that challenges the musician, but simply the musician's ability to move to them quickly enough. Because of this and its complexity, it requires a great deal of skill to perform.