DB performance issues by heavy ISC SQL queries

Dear Thomas,

first of all we really like the ISC plugin and are using it since beta. But, due to our latest performance review of or server we have figured out that ISC does a lot of heavy database work by running some SQL queries over and over. We don’t really understand why this happens and runs so often. Sure, we can see it is a heavy join… but can you help by answering a free questions?

• What does this SQL query do?
• Why it might run over and over on our machine?
• Do you have any tips to improve the SQL load in your plugin?

Our WP data:

• ~8.000 post articles
• ~24,755 media items

SQL Query as Code:

´SELECT   xyz_wp_posts.ID
					 FROM xyz_wp_posts  
                LEFT JOIN xyz_wp_postmeta ON ( xyz_wp_posts.ID = xyz_wp_postmeta.post_id )  
                LEFT JOIN xyz_wp_postmeta AS mt1 ON ( xyz_wp_posts.ID = mt1.post_id )  
                LEFT JOIN xyz_wp_postmeta AS mt2 ON ( xyz_wp_posts.ID = mt2.post_id AND mt2.meta_key = 'isc_image_source' )
					 WHERE 1=1  AND (
  (
    ( xyz_wp_postmeta.meta_key = 'isc_image_source' AND xyz_wp_postmeta.meta_value = '' )
    AND
    ( mt1.meta_key = 'isc_image_source_own' AND mt1.meta_value != '1' )
  )
  OR
  mt2.post_id IS NULL
) AND xyz_wp_posts.post_type = 'attachment' AND ((xyz_wp_posts.post_status = 'inherit'))
					 GROUP BY xyz_wp_posts.ID
					 ORDER BY xyz_wp_posts.post_date DESC
					 LIMIT 0, 100

SQL Query as Screenshot:
https://ibb.co/yV49DB7