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 (
    21604, 21607, 21606, 21602, 21599, 21617, 
    21616, 21611, 21595, 21597, 21578, 
    21596, 21577, 21561, 21562, 21563, 
    21610, 21615, 21601, 21598, 21591, 
    21592, 21593, 21594
  )

Query time 0.00068

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "f",
      "access_type": "ALL",
      "possible_keys": ["idx_group_id"],
      "rows": 6,
      "filtered": 100
    },
    "table": {
      "table_name": "g",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["id"],
      "ref": ["admin_test.f.group_id"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "p",
      "access_type": "ALL",
      "possible_keys": ["PRIMARY", "idx_group_id"],
      "rows": 15,
      "filtered": 80,
      "attached_condition": "p.group_id = f.group_id and p.product_id in (21604,21607,21606,21602,21599,21617,21616,21611,21595,21597,21578,21596,21577,21561,21562,21563,21610,21615,21601,21598,21591,21592,21593,21594)"
    }
  }
}