A good instructor should take approximately an hour plus to prepare for each class. While they may know the material class presentation may be different based on the student population. Review the the material prior to class enable the instructor to be better prepared to meet the challenges of the class. Failure to prepare ahead could result in materials being covered a second in error. The instructor is there to help the students develop an understanding of the course material failure to do so result in students not understanding information and course failure.