SQL select query too slow on webhost, fine on localhost - sql

Question

SELECT c.customers_lastname,
cg.customers_group_name,
dctc.coupons_id AS coupId,
dcto.coupons_id AS coupIdUsed,
dc.coupons_date_start AS coupStart,
count(DISTINCT o.orders_id) AS totalorders,
sum(op.products_quantity * op.final_price) AS ordersum
from
customers c LEFT JOIN customers_groups cg ON cg.customers_group_id = c.customers_group_id
LEFT JOIN (discount_coupons_to_customers dctc
LEFT JOIN discount_coupons dc ON dc.coupons_id = dctc.coupons_id
LEFT JOIN discount_coupons_to_orders dcto ON dcto.coupons_id = dctc.coupons_id
) ON c.customers_id = dctc.customers_id, orders_products op, orders o
WHERE c.customers_id = o.customers_id
AND c.customers_promotions = '0'
AND o.orders_id = op.orders_id
GROUP BY c.customers_id
ORDER BY ordersum DESC LIMIT 0, 10
The above query returns all customers that ever bought anything in our webshop (and some extra data), sorted by total order amount. It runs fine on localhost (a few seconds) but takes up to a minute on remote server. To make matters worse, the query can be modified via a form to include extra bits in the GROUP BY clause like:
HAVING (sum(op.products_quantity * op.final_price) >= 1000
AND/OR count(DISTINCT o.orders_id) > 2)
which doesn't exactly speed things up. there's about 5000 customers and 3000 orders at present. I added a time constraint WHERE order not older than one year but things didnt speed up after that.
i compared my local server and the online one.
localhost linux kernel is 3.2, online 2.6,
localhost php 5.4.4, online 5.3.26,
localhost mysql 5.5, online 5.1,
localhost php memory limit 128M, online 126M.
is there an obvious bottleneck? i sent an email to my webhost but didnt get a response. if I need to swap hosts I will but would like to know what to look out for. cheers,
Edit
using explain: (not sure how to format this, and no idea what it means) returns
`+----+-------------+-------+--------+-----------------------------+--------------+---------+---------------------+-------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+-----------------------------+--------------+---------+---------------------+-------+----------------------------------------------+
| 1 | SIMPLE | c | ALL | PRIMARY | NULL | NULL | NULL | 5541 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | cp | eq_ref | PRIMARY | PRIMARY | 4 | rpc.c.customers_id | 1 | |
| 1 | SIMPLE | dctc | eq_ref | customers_id,customers_id_2 | customers_id | 4 | rpc.c.customers_id | 1 | |
| 1 | SIMPLE | dcto | ref | PRIMARY | PRIMARY | 34 | rpc.dctc.coupons_id | 0 | Using index |
| 1 | SIMPLE | dc | ALL | PRIMARY | NULL | NULL | NULL | 1 | |
| 1 | SIMPLE | cg | ALL | PRIMARY | NULL | NULL | NULL | 5 | Using where; Using join buffer |
| 1 | SIMPLE | o | ALL | PRIMARY | NULL | NULL | NULL | 5010 | Using where; Using join buffer |
| 1 | SIMPLE | op | ALL | NULL | NULL | NULL | NULL | 10675 | Using where; Using join buffer |
+----+-------------+-------+--------+-----------------------------+--------------+---------+---------------------+-------+----------------------------------------------+`