Delete all posts of a custom post type—efficiently

You can delete all post via $wpdb

DELETE FROM wp_posts WHERE post_type="post_type";
DELETE FROM wp_postmeta WHERE post_id NOT IN (SELECT id FROM wp_posts);
DELETE FROM wp_term_relationships WHERE object_id NOT IN (SELECT id FROM wp_posts)

or use this query replace it with {{your CPT}} with your Custom Post Type

DELETE a,b,c
    FROM wp_posts a
    LEFT JOIN wp_term_relationships b
        ON (a.ID = b.object_id)
    LEFT JOIN wp_postmeta c
        ON (a.ID = c.post_id)
    WHERE a.post_type="{{your CPT}}";