We just discovered an issue where a user who is enrolled in multiple categories is experiencing extreme slowdowns while the calendar block or upcoming events block loads. The user is enrolled in 2419 courses in Moodle through category enrollments... This is an academic dean role that includes all of his historic courses. We found that category enrollments physically enroll the user in the course and therefore significantly impact the course load time when a calendar block or upcoming events block are in a course. 

When the user loads a course there is a significant time impact of over 10 seconds waiting for the course calendar or upcoming events block to populate. When the course calendar block was hidden or permissions removed the time decreases to less than a second.