The course overview block provides a link to create a new course if the user has course create capability.  The get_nearest_editable_subcategory() function is used to get the course category the user has access to.  This behavior was introduced in this commit: https://github.com/moodle/moodle/commit/89849347e5adc6585e3e6a26b535f1da90226d3f 

This seems to create a problem for sites with a large number of course categories (2700 in our instance).  The result is the dashboard with the myoverview block taking ~10 seconds to load which leads to a poor experience for the users.