Very slow query when too many joins

[As Os]

In my db I have 2 tables:
1.table 'fields_values' with fields: id,field_id(FK to fields table), ads_id(FK to ads table), value.
2.table 'ads' with fields: id,field_id (which actually is in a json format field).

My query is like this:



SELECT DISTINCT a.*,b.value
FROM ads as a

LEFT JOIN fields_values AS b
ON (`b`.`ads_id` = `a`.`id` AND `b`.field_id="183")

LEFT JOIN fields_values AS c
ON (`c`.`ads_id` = `a`.`id` AND `c`.field_id="183")

Up to 25 joins, it's ok.
But on 26, 27 joins it slows down.
On more joins, it slows down dramatically.

How can I fix this?
(I tried to post the EXPLAIN from phpmyadmin but I failed)