SELECT 
  a.object_id, 
  COUNT(b.post_id) as total_items 
FROM 
  cscart_discussion as a 
  LEFT JOIN cscart_discussion_posts as b ON a.thread_id = b.thread_id 
WHERE 
  a.object_type = 'M' 
  AND b.status = 'A' 
GROUP BY 
  a.object_id

Query time 0.00105

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE b ALL thread_id,thread_id_2 4 Using where; Using temporary; Using filesort
1 SIMPLE a eq_ref PRIMARY PRIMARY 3 hobilifedt.b.thread_id 1 Using where