SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    315, 318, 311, 314, 310, 316, 309, 317, 
    319, 312, 313, 308
  ) 
  AND category_id != 265

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "parent", "p_category_id"],
      "key": "parent",
      "key_length": "3",
      "used_key_parts": ["parent_id"],
      "rows": 12,
      "filtered": 100,
      "index_condition": "cscart_categories.parent_id in (315,318,311,314,310,316,309,317,319,312,313,308)",
      "attached_condition": "cscart_categories.category_id <> 265"
    }
  }
}