Hi Thomschu,
thank you for reporting this. I don’t really know about the load issues. Maybe you could report an complete issue @ GitHub · Where software is built with a detailed description of your setup?
Did you try ownCloud Documentation Overview
and here especially?
sudo -u www-data php occ ldap:show-remnants displays a table with all users that have been marked as deleted, and their LDAP data.
sudo -u www-data php occ user:delete [user]
Please also note that the user_ldap is currently under a major rework and a new improved version should be available in 3-4 weeks.