Steps to reproduce the issue

System with large number of guest sessions very slow.
Also query does not use ORDER BY so no guarantee last X logged in users will be shown.

Expected result

Actual result

System information (as much as possible)

Joomla 3.8.10
MySQL 5.5.52

Additional comments

Suggest changing WHERE clause in getList() method in ModLoggedHelper to
$query = $db->getQuery(true)
->select('s.time, s.client_id, u.id, u.name, u.username')
->from('#__session AS s')
->join('LEFT', '#__users AS u ON s.userid = u.id')
->where('s.userid > 0') // Added, uses existing index
->where('s.guest = 0')
->order('s.time DESC'); // Added, guarantees last X rows returned