SELECT 
  n.sf_id, 
  n.category_id, 
  n.features_hash, 
  nd.name_in_subcategory_block 
FROM 
  cscart_ab__sf_names as n 
  INNER JOIN cscart_ab__sf_name_descriptions as nd ON (
    nd.sf_id = n.sf_id 
    AND nd.lang_code = 'ru'
  ) 
WHERE 
  n.category_id = 499 
  AND n.fixed in ('F', 'U') 
  AND n.show_in_subcategory_block = 'Y' 
ORDER BY 
  n.position_in_subcategory_block ASC, 
  nd.name_in_subcategory_block ASC

Query time 0.00031

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "n.position_in_subcategory_block, nd.name_in_subcategory_block",
      "temporary_table": {
        "table": {
          "table_name": "n",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "fixed_category_hash"],
          "key": "fixed_category_hash",
          "key_length": "7",
          "used_key_parts": ["fixed", "category_id"],
          "rows": 1,
          "filtered": 100,
          "index_condition": "n.category_id = 499 and n.`fixed` in ('F','U')",
          "attached_condition": "n.show_in_subcategory_block = 'Y'"
        },
        "table": {
          "table_name": "nd",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "10",
          "used_key_parts": ["sf_id", "lang_code"],
          "ref": ["devmotorsbay.n.sf_id", "const"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "nd.lang_code = 'ru'"
        }
      }
    }
  }
}