SELECT 
  CONCAT(u.firstname, ' ', u.lastname) 
FROM 
  cscart_blog_authors as b 
  LEFT JOIN cscart_users as u ON b.user_id = u.user_id 
WHERE 
  b.page_id = 58

Query time 0.00090

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.40"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "b",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "page_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "1.00",
            "eval_cost": "0.20",
            "prefix_cost": "1.20",
            "data_read_per_join": "8"
          },
          "used_columns": [
            "page_id",
            "user_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "u",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "user_id"
          ],
          "key_length": "3",
          "ref": [
            "webmarco.b.user_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "1.00",
            "eval_cost": "0.20",
            "prefix_cost": "2.40",
            "data_read_per_join": "6K"
          },
          "used_columns": [
            "user_id",
            "firstname",
            "lastname"
          ]
        }
      }
    ]
  }
}