SELECT 
  CONCAT(u.firstname, ' ', u.lastname) as author, 
  b.page_id 
FROM 
  cscart_blog_authors as b 
  LEFT JOIN cscart_users as u ON b.user_id = u.user_id 
WHERE 
  b.page_id IN (7, 8, 10, 9)

Query time 0.00043

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.61"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "b",
          "access_type": "index",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "page_id",
            "user_id"
          ],
          "key_length": "6",
          "rows_examined_per_scan": 4,
          "rows_produced_per_join": 4,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "2.01",
            "eval_cost": "0.80",
            "prefix_cost": "2.81",
            "data_read_per_join": "32"
          },
          "used_columns": [
            "page_id",
            "user_id"
          ],
          "attached_condition": "(`satishecarter_dec_setup_001`.`b`.`page_id` in (7,8,10,9))"
        }
      },
      {
        "table": {
          "table_name": "u",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "user_id"
          ],
          "key_length": "3",
          "ref": [
            "satishecarter_dec_setup_001.b.user_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 4,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "4.00",
            "eval_cost": "0.80",
            "prefix_cost": "7.61",
            "data_read_per_join": "25K"
          },
          "used_columns": [
            "user_id",
            "firstname",
            "lastname"
          ]
        }
      }
    ]
  }
}