GetAvailableSerialsOfEsd function is very slow

[mitelg]

sOrder::getAvailableSerialsOfEsd

starts working slowly when the number of the license keys in DB increases and that causes the clients to wait about 1 to 2 minutes in order to receive the order. (the number of purchased product keys in DB is about 20,000). These all is caused by LEFT JOIN query in SQL.
Suggestion: Replace the LEFT JOIN with another code given bellow.

return $this->db->fetchAll(
    'SELECT s_articles_esd_serials.id AS id, s_articles_esd_serials.serialnumber AS serialnumber
     FROM s_articles_esd_serials
     WHERE NOT s_articles_esd_serials.id IN (SELECT serialID FROM s_order_esd)
     AND s_articles_esd_serials.esdID= :esdId', 
    ['esdId' => $esdId]
);