SELECT 
  product_id, 
  amount, 
  destination_id 
FROM 
  cscart_warehouses_destination_products_amount 
WHERE 
  product_id IN(821, 826) 
  AND storefront_id = 0

Query time 0.00016

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00345563,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_warehouses_destination_products_amount",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "idx_storefront_id"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "loops": 1,
          "rows": 1,
          "cost": 0.00345563,
          "filtered": 100,
          "attached_condition": "cscart_warehouses_destination_products_amount.storefront_id = 0 and cscart_warehouses_destination_products_amount.product_id in (821,826)"
        }
      }
    ]
  }
}