There are some pages, like the course participants and the list of site users that take a long time to load.

Likely the cause is the number of records in the user table, more than 4M, being about 1M deleted users.

The first approach was to delete the records of the deleted users, but the problem with dropping the records is that there could be a lot of queries joining the "user" table, which could lead to content loss (orphan) or errors due to a not-found record.