SELECT 
  cscart_products.* 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    603, 604, 261, 262, 259, 260, 403, 404, 
    265, 266, 263, 264, 405, 406, 267, 268
  )

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_products",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_products.product_id in (603,604,261,262,259,260,403,404,265,266,263,264,405,406,267,268)"
    }
  }
}