SELECT 
  cscart_product_prices.product_id, 
  MIN(
    IF(
      cscart_product_prices.percentage_discount = 0, 
      cscart_product_prices.price, 
      cscart_product_prices.price - (
        cscart_product_prices.price * cscart_product_prices.percentage_discount
      )/ 100
    )
  ) AS price 
FROM 
  cscart_product_prices 
WHERE 
  cscart_product_prices.product_id IN (
    15959, 15960, 15961, 15962, 10100, 10175, 
    10101, 10644, 10606, 10177, 10178, 
    10643, 10102, 9940, 15965, 15966, 9905, 
    9904, 10041, 10053
  ) 
  AND cscart_product_prices.lower_limit = 1 
  AND cscart_product_prices.usergroup_id IN (0, 1) 
GROUP BY 
  cscart_product_prices.product_id

Query time 0.00045

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "cscart_product_prices.product_id",
      "temporary_table": {
        "table": {
          "table_name": "cscart_product_prices",
          "access_type": "range",
          "possible_keys": [
            "usergroup",
            "product_id",
            "lower_limit",
            "usergroup_id"
          ],
          "key": "usergroup_id",
          "key_length": "6",
          "used_key_parts": ["usergroup_id", "product_id"],
          "rows": 40,
          "filtered": 100,
          "index_condition": "cscart_product_prices.product_id in (15959,15960,15961,15962,10100,10175,10101,10644,10606,10177,10178,10643,10102,9940,15965,15966,9905,9904,10041,10053) and cscart_product_prices.usergroup_id in (0,1)",
          "attached_condition": "cscart_product_prices.lower_limit = 1"
        }
      }
    }
  }
}