These two small queries are repeated until they generate ~6000 lines of SQL when viewing the contact activity log (probably elsewhere).

civicrm-sql-optimization.sql

-- this query is repeated several hundreds of times, with f = [3..437]

SELECT     f.id, f.name AS field_name, f.label AS field_label, g.name AS group_name, g.title AS group_title
FROM       civicrm_custom_field f
INNER JOIN civicrm_custom_group g ON f.custom_group_id = g.id
WHERE      f.id IN (3)

-- this query is similarly repeated hundreds of times, with parent_id = [NULL..285]

SELECT id, label, url, permission, permission_operator, has_separator, parent_id, is_active, name
FROM civicrm_navigation
WHERE  parent_id IS NULL
AND domain_id = 1
ORDER BY parent_id, weight