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 (
    1922, 1855, 1920, 1469, 344, 1850, 1551, 
    284, 1475, 1276, 1569, 1198, 1179, 1281, 
    926, 1258, 1168, 1394, 1259, 1865, 390, 
    1204, 1864, 1863
  ) 
  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 0.00126

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_prices",
      "access_type": "range",
      "possible_keys": ["usergroup", "product_id", "lower_limit", "usergroup_id"],
      "key": "product_id",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 26,
      "filtered": 4.897957802,
      "index_condition": "cscart_product_prices.product_id in (1922,1855,1920,1469,344,1850,1551,284,1475,1276,1569,1198,1179,1281,926,1258,1168,1394,1259,1865,390,1204,1864,1863)",
      "attached_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.usergroup_id in (0,1)"
    }
  }
}