How to increase performance with joins to create VIEW from Wordpress wp_usermeta
In this case you don't need to join the table to itself.  If you only query it once then it will go faster.

SELECT user_id, 
    MAX(CASE WHEN meta_key='vorname' THEN meta_value ELSE null END) Vorname_M,
    MAX(CASE WHEN meta_key='nachname' THEN meta_value ELSE null END) Nachname_M,
    MAX(CASE WHEN meta_key='adress' THEN meta_value ELSE null END) Adresse_M
FROM wp_usermeta
GROUP BY user_id