To enable slow query logging, add 'logslow' => 1 to the 'dboptions' array in config.php.
This will cause queries over 1 second (the 1 can be changed to any number of seconds) to be logged in the 'log_queries' table in the database. This table can also be export and sent to Totara Support.
Only enable slow query logging long enough to test slow queries and immediately disable after testing.