Optimize Query - Select COUNT and SUM from the same subquery

The problem with your query is that you have nested correlated sub queries. Correlated sub queries are a huge hit to SQL performance. In this case you want to try use joins with aggregation (group by).

I don't understand the intricacies of the schema your database, so I'm not quite sure why you're limiting the sub queries for firstName etc to 1 record. I can edit the answer to accommodate that if you give more detail.

I would suggest you try something like this:

SELECT   u.ID,
         u.user_email AS mail, 
         u.user_login AS userName, 
         u.user_registered  AS signUpDate, 
         mfn.meta_value as firstName, 
         mln.meta_value as lastName, 
         mbp.meta_value as billingPhone, 
         msp.meta_value as shippingPhone, 
         COUNT(pval.meta_value) as orderCount,
         SUM(pval.meta_value) as moneySpent 
FROM     wp_users u
JOIN     wp_usermeta as mfn  ON u.ID = mfn.user_id  AND mfn.meta_key = 'first_name'
JOIN     wp_usermeta as mln  ON u.ID = mln.user_id  AND mln.meta_key = 'last_name'
JOIN     wp_usermeta as mbp  ON u.ID = mbp.user_id  AND mbp.meta_key = 'billing_phone'
JOIN     wp_usermeta as msp  ON u.ID = msp.user_id  AND msp.meta_key = 'shipping_phone'
JOIN     wp_postmeta as p    ON p.meta_value = u.ID AND p.meta_key = '_customer_user'
JOIN     wp_postmeta as pval ON pval.post_id = p.post_id AND pval.meta_key = '_order_total'
GROUP BY u.ID,
         u.user_email, 
         u.user_login, 
         u.user_registered, 
         mfn.meta_value, 
         mln.meta_value, 
         mbp.meta_value, 
         msp.meta_value 

As per @O.Jones's comment, consider using LEFT JOINS instead of INNER JOINS so that the query still returns results where a meta_value is missing.

@Timur, as per your question in the comments of the first answer; you'd have to do something like this. You still need joins but you don't need as many for the wp_usermeta table. You will still need to join to wp_postmeta twice, because you can't do a join on the '_customer_user' meta_key and simultaneously retrieve the '_order_total' meta_key in a single join.

Just a note on the MAX(CASE WHEN um.meta_key = '...' THEN um.meta_value END) logic; it's the equivalent to MAX(CASE WHEN um.meta_key = '...' THEN um.meta_value ELSE NULL END).

SELECT    u.ID, 
          u.user_email AS mail,
          u.user_login AS userName,
          u.user_registered AS signUpDate,
          MAX(CASE WHEN um.meta_key = 'first_name'     THEN um.meta_value END) AS firstName, 
          MAX(CASE WHEN um.meta_key = 'last_name'      THEN um.meta_value END) AS lastName, 
          MAX(CASE WHEN um.meta_key = 'billing_phone'  THEN um.meta_value END) AS billingPhone, 
          MAX(CASE WHEN um.meta_key = 'shipping_phone' THEN um.meta_value END) AS shippingPhone,
          COUNT(pval.meta_value) AS orderCount,
          SUM(pval.meta_value) AS moneySpent
FROM      wp_users AS u
LEFT JOIN wp_usermeta AS um   ON u.ID = um.user_id
LEFT JOIN wp_postmeta AS pm   ON u.ID = pm.meta_value      AND pm.meta_key = '_customer_user'
LEFT JOIN wp_postmeta AS pval ON pm.post_id = pval.post_id AND pval.meta_key = '_order_total'
WHERE     (um.meta_key IN ('first_name', 'last_name', 'billing_phone', 'shipping_phone') 
           OR um.meta_key IS NULL)
GROUP BY  u.ID, 
          u.user_email,
          u.user_login,
          u.user_registered;

I'm not 100% sure on the logic. Let me know if it needs tweaking

Here subquery has been introduced for minimizing multiple times using a single table and removing few column from GROUP BY clause for optimization purpose. Use WHERE clause in every subquery if need a particular user/ few users or any searching condition. If subquery returns no data but needed user info then use LEFT JOIN instead of INNER JOIN.

-- MySQL
SELECT u.ID
     , u.user_email AS mail
     , u.user_login AS userName
     , u.user_registered  AS signUpDate
     , t.firstName
     , t.lastName
     , t.billingPhone
     , t.shippingPhone
     , r.orderCount
     , r.moneySpent
FROM wp_users u      
INNER JOIN (SELECT user_id
                 , MAX(CASE WHEN meta_key = 'first_name' THEN meta_value END) firstName 
                 , MAX(CASE WHEN meta_key = 'last_name' THEN meta_value END) lastName 
                 , MAX(CASE WHEN meta_key = 'billing_phone' THEN meta_value END) billingPhone 
                 , MAX(CASE WHEN meta_key = 'shipping_phone' THEN meta_value END) shippingPhone 
            FROM wp_usermeta 
            WHERE meta_key IN ('first_name', 'last_name', 'billing_phone', 'shipping_phone') 
            GROUP BY user_id) t
       ON u.ID = t.user_id
INNER JOIN (SELECT p.meta_value
                 , COUNT(pval.meta_value) as orderCount
                 , SUM(pval.meta_value) as moneySpent 
            FROM wp_postmeta as p
            INNER JOIN wp_postmeta as pval
                    ON p.post_id = pval.post_id
                   AND p.meta_key = '_customer_user'
                   AND pval.meta_key = '_order_total'
            GROUP BY p.meta_value) r
        ON u.ID = r.meta_value;