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 (
    19938, 19939, 19940, 19941, 19943, 19600, 
    19601, 18570, 14106, 14070, 14045, 
    14051, 14044, 14097, 14093, 14105, 
    14125, 18488, 18489, 18600, 18603
  ) 
  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 0.00092

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.054455256,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_prices",
          "access_type": "range",
          "possible_keys": [
            "usergroup",
            "product_id",
            "lower_limit",
            "usergroup_id"
          ],
          "key": "usergroup",
          "key_length": "9",
          "used_key_parts": ["product_id", "usergroup_id", "lower_limit"],
          "loops": 1,
          "rows": 42,
          "cost": 0.0410523,
          "filtered": 49.95539856,
          "attached_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.product_id in (19938,19939,19940,19941,19943,19600,19601,18570,14106,14070,14045,14051,14044,14097,14093,14105,14125,18488,18489,18600,18603) and cscart_product_prices.usergroup_id in (0,1)"
        }
      }
    ]
  }
}