We've been seeing some massive speed issues on the /my page and they largely seem to be a result of get_fast_modinfo repeatedly resetting.
Having tracked this issue back:

• blocks/course_overview/block_course_overview.php::get_content()
  • calls enrol_get_my_courses() to retrieve a list of courses
  • passes this list to print_overview($courses, $remote_courses)
• course/lib.php::print_overview()
  • loops through each module, and calls its {$mod->name})print_overview() function
• {$mod->name})print_overview() calls get_all_instances_in_courses()
• lib/datalib.php::get_all_instances_in_courses() calls get_fast_modinfo()
• lib/modinfolib.php::get_fast_modinfo()
  • checks whether the cache is filled and a relevant entry is found for the specified course
  • if found returns
  • if not inserts a new course_modinfo object into the cache
• lib/modinfolib.php::course_modinfo
  • checks whether the supplied course has a valid modinfo and sectioncache - if not, it calls rebuild_course_cache which resets the get_fast_modinfo cache

Unfortunately though, the course_overview block doesn't retrieve the sectioncache field for each course from the DB, and as a result, every time a course is retrieved, the entire get_fast_modinfo cache is emptied.