Time is a huge factor in not only preparing lessons but in carrying out those cooperative group lessons. Deciding how much time to allot each activity is difficult. A highly efficient group can take much less time than a group whose members have different ideas, a variety of approaches, no ideas, members who lack necessary background skills and concepts, or slow work rate.
Students are more efficient at spreading understanding than teachers.
I tried teaching a beginning Calculus class with the structure completely founded on the cooperative learning concept. I was not pleased with the result. It could possibly be that some of the students were not congenial or did not have a good mathematical background. Works particularly well in math for elementary ed majors courses. I m totally sold on the approach and use this approach in every course I teach.