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 = 3445 
  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 0.00215

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.70"
    },
    "ordering_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "nested_loop": [
        {
          "table": {
            "table_name": "nd",
            "access_type": "ALL",
            "possible_keys": [
              "PRIMARY"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 1,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "0.10",
              "prefix_cost": "0.35",
              "data_read_per_join": "6K"
            },
            "used_columns": [
              "sf_id",
              "lang_code",
              "name_in_subcategory_block"
            ],
            "attached_condition": "(`s2cart`.`nd`.`lang_code` = 'en')"
          }
        },
        {
          "table": {
            "table_name": "n",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "fixed_category_hash"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "sf_id"
            ],
            "key_length": "4",
            "ref": [
              "s2cart.nd.sf_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 1,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "0.10",
              "prefix_cost": "0.70",
              "data_read_per_join": "336"
            },
            "used_columns": [
              "sf_id",
              "category_id",
              "features_hash",
              "fixed",
              "show_in_subcategory_block",
              "position_in_subcategory_block"
            ],
            "attached_condition": "((`s2cart`.`n`.`category_id` = 3445) and (`s2cart`.`n`.`fixed` in ('F','U')) and (`s2cart`.`n`.`show_in_subcategory_block` = 'Y'))"
          }
        }
      ]
    }
  }
}