SELECT 
  pf.feature_id, 
  pf.feature_style, 
  pf.position, 
  pf.purpose, 
  pf.display_on_catalog, 
  pfd.description, 
  pfd.internal_name, 
  pfd.prefix, 
  pfd.suffix, 
  (
    CASE WHEN pf.purpose = 'group_catalog_item' THEN 0 ELSE 1 END
  ) AS purpose_position 
FROM 
  cscart_product_features AS pf 
  INNER JOIN cscart_product_features_descriptions AS pfd ON pf.feature_id = pfd.feature_id 
  AND pfd.lang_code = 'ja' 
WHERE 
  pf.feature_id IN (9, 7, 8) 
ORDER BY 
  purpose_position ASC, 
  pf.position ASC, 
  pf.feature_id ASC

Query time 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "read_sorted_file": {
      "filesort": {
        "sort_key": "case when pf.purpose = 'group_catalog_item' then 0 else 1 end, pf.position, pf.feature_id",
        "table": {
          "table_name": "pf",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["feature_id"],
          "rows": 3,
          "filtered": 100,
          "index_condition": "pf.feature_id in (9,7,8)"
        }
      }
    },
    "table": {
      "table_name": "pfd",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["feature_id", "lang_code"],
      "ref": ["cscart_purchase_fl_com.pf.feature_id", "const"],
      "rows": 1,
      "filtered": 100,
      "index_condition": "pfd.lang_code = 'ja'"
    }
  }
}