Deleting a course will be quite slow, because Moodle deletes everything belonging to the course.
However, 5-10 minutes seems excessive.
If you can be bothered, try to find some way of logging what is going on. For example, can your database server be set to log every query it executes, with the time it took to run? If so, does anything there look anomalously slow?
Another option is this approach: http://tjhunt.blogspot.co.uk/2013/05/performance-testing-moodle.html