SELECT 
  f.feature_id, 
  f.purpose, 
  p.product_id, 
  p.parent_product_id, 
  g.id, 
  g.code 
FROM 
  cscart_product_variation_group_features AS f 
  INNER JOIN cscart_product_variation_groups AS g ON f.group_id = g.id 
  INNER JOIN cscart_product_variation_group_products AS p ON f.group_id = p.group_id 
WHERE 
  p.product_id IN (42, 39, 40, 37, 43)

Query time 0.00095

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "10.34"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "g",
          "access_type": "ALL",
          "possible_keys": [
            "PRIMARY"
          ],
          "rows_examined_per_scan": 2,
          "rows_produced_per_join": 2,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "2.02",
            "eval_cost": "0.40",
            "prefix_cost": "2.42",
            "data_read_per_join": "800"
          },
          "used_columns": [
            "id",
            "code"
          ]
        }
      },
      {
        "table": {
          "table_name": "p",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "idx_group_id"
          ],
          "key": "idx_group_id",
          "used_key_parts": [
            "group_id"
          ],
          "key_length": "3",
          "ref": [
            "atulecarter_atul_demo3.g.id"
          ],
          "rows_examined_per_scan": 2,
          "rows_produced_per_join": 1,
          "filtered": "45.45",
          "cost_info": {
            "read_cost": "4.00",
            "eval_cost": "0.36",
            "prefix_cost": "7.22",
            "data_read_per_join": "29"
          },
          "used_columns": [
            "product_id",
            "parent_product_id",
            "group_id"
          ],
          "attached_condition": "(`atulecarter_atul_demo3`.`p`.`product_id` in (42,39,40,37,43))"
        }
      },
      {
        "table": {
          "table_name": "f",
          "access_type": "ALL",
          "possible_keys": [
            "idx_group_id"
          ],
          "rows_examined_per_scan": 3,
          "rows_produced_per_join": 1,
          "filtered": "33.33",
          "using_join_buffer": "Block Nested Loop",
          "cost_info": {
            "read_cost": "2.03",
            "eval_cost": "0.36",
            "prefix_cost": "10.34",
            "data_read_per_join": "189"
          },
          "used_columns": [
            "feature_id",
            "purpose",
            "group_id"
          ],
          "attached_condition": "(`atulecarter_atul_demo3`.`f`.`group_id` = `atulecarter_atul_demo3`.`g`.`id`)"
        }
      }
    ]
  }
}