似乎您有一个无用的SELECT DISTINCT..您是否正在寻找计数(DISTINCT..)
这样，您就可以避免在Main SELECT中为每一行嵌套SELECT。

SELECT  c.id as "campaign_id",
        c.created_by_user,
        c.name,
        c.date_added,
        c.date_modified,
        COUNT(DISTINCT cl.lead_id) as number_of_leads,
        GROUP_CONCAT(lt.tag) as tags,
        cat.title as category_name,
        GROUP_CONCAT(ll.name) as segment_name,
        GROUP_CONCAT(emails.name) as email_name,
        CASE WHEN c.is_published = 1 THEN "Yes" ELSE "No" END AS "published",
        CASE WHEN c.publish_down > now() THEN "Yes" 
             WHEN c.publish_down > now() AND c.is_published = 0 THEN "Yes" 
             ELSE "No" END AS "expired"
FROM campaigns c 
    LEFT JOIN campaign_leads cl ON cl.campaign_id = c.id
    LEFT JOIN lead_tags_xref ltx on cl.lead_id = ltx.lead_id 
    LEFT JOIN lead_tags lt on ltx.tag_id = lt.id 
    LEFT JOIN categories cat on c.category_id = cat.id 
    LEFT JOIN lead_lists_leads llist on cl.lead_id = llist.lead_id 
    LEFT JOIN lead_lists ll on llist.leadlist_id = ll.id 
    LEFT JOIN email_list_xref el on ll.id = el.leadlist_id 
    LEFT JOIN emails on el.email_id = emails.id 
GROUP BY c.id;

无论如何，请确保您有一个适当的综合指数

table campaign_leads columns campaign_id, lead_id
table lead_tags_xref columns lead_id, tag_id
table lead_lists_leads columns lead_id, leadlist_id
table email_list_xref columns leadlist_id, email_id