Nine times out of ten this is down to a table that hasn’t been indexed properly.

Use 

SHOW PROCESSLIST;

to see your MySQL processes. Check out the ‘time’ column.

To add an index use:

ALTER TABLE `table` ADD INDEX `product_id` (`product_id`)

http://stackoverflow.com/questions/3002605/how-to-add-indexes-to-mysql-tables

and

http://dev.mysql.com/doc/refman/5.0/en/create-index.html