SELECT 
  SQL_CALC_FOUND_ROWS cpqq.*, 
  product 
FROM 
  ushop_csc_product_questions_posts as cpqq 
  LEFT JOIN ushop_product_descriptions ON ushop_product_descriptions.product_id = cpqq.product_id 
  LEFT JOIN ushop_csc_product_questions_posts as cpqa ON cpqa.parent_id = cpqq.post_id 
WHERE 
  cpqq.parent_id = 0 
  AND ushop_product_descriptions.lang_code = 'es' 
  AND cpqq.product_id = 558 
  AND cpqa.parent_id IS NOT NULL 
  AND cpqa.status IN ('A') 
  AND cpqq.status IN ('A') 
ORDER BY 
  cpqq.post_id desc 
LIMIT 
  0, 50

Query time 0.00225

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.30"
    },
    "ordering_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.20"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "ushop_product_descriptions",
            "access_type": "const",
            "possible_keys": [
              "PRIMARY",
              "product_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id",
              "lang_code"
            ],
            "key_length": "9",
            "ref": [
              "const",
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 1,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "0.00",
              "eval_cost": "0.10",
              "prefix_cost": "0.00",
              "data_read_per_join": "6K"
            },
            "used_columns": [
              "product_id",
              "lang_code",
              "product"
            ]
          }
        },
        {
          "table": {
            "table_name": "cpqa",
            "access_type": "ALL",
            "rows_examined_per_scan": 5,
            "rows_produced_per_join": 1,
            "filtered": "20.00",
            "cost_info": {
              "read_cost": "0.65",
              "eval_cost": "0.10",
              "prefix_cost": "0.75",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "post_id",
              "parent_id",
              "status"
            ],
            "attached_condition": "(((`marketplace`.`cpqa`.`parent_id` is not null) and (`marketplace`.`cpqa`.`status` = 'A')) and (`marketplace`.`cpqa`.`parent_id` is not null))"
          }
        },
        {
          "table": {
            "table_name": "cpqq",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "post_id"
            ],
            "key_length": "3",
            "ref": [
              "marketplace.cpqa.parent_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "20.00",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "0.02",
              "prefix_cost": "1.10",
              "data_read_per_join": "393"
            },
            "used_columns": [
              "post_id",
              "parent_id",
              "id_path",
              "product_id",
              "company_id",
              "timestamp",
              "status",
              "user_id",
              "user_name",
              "email",
              "lang_code",
              "post",
              "email_timestamp"
            ],
            "attached_condition": "((`marketplace`.`cpqq`.`product_id` = 558) and (`marketplace`.`cpqq`.`parent_id` = 0) and (`marketplace`.`cpqq`.`status` = 'A'))"
          }
        }
      ]
    }
  }
}