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 = 'en'
  ) 
WHERE 
  n.category_id = 422 
  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 0.00068

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "n.position_in_subcategory_block, nd.name_in_subcategory_block",
      "temporary_table": {
        "nested_loop": [
          {
            "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": 89,
              "filtered": 100,
              "index_condition": "n.category_id = 422 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": ["lillyshop_lillyshop.n.sf_id", "const"],
              "rows": 1,
              "filtered": 100,
              "attached_condition": "nd.lang_code = 'en'"
            }
          }
        ]
      }
    }
  }
}