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 0.00034

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.129296417,
    "filesort": {
      "sort_key": "a.object_id",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "b",
              "access_type": "index",
              "possible_keys": ["thread_id", "thread_id_2"],
              "key": "thread_id_2",
              "key_length": "6",
              "used_key_parts": ["thread_id", "status"],
              "loops": 1,
              "rows": 49,
              "cost": 0.013833485,
              "filtered": 100,
              "attached_condition": "b.`status` = 'A'",
              "using_index": true
            }
          },
          {
            "table": {
              "table_name": "a",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["thread_id"],
              "ref": ["u985510652_ecartify.b.thread_id"],
              "loops": 49,
              "rows": 1,
              "cost": 0.08396836,
              "filtered": 100,
              "attached_condition": "a.object_type = 'M'"
            }
          }
        ]
      }
    }
  }
}