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 (
    20838, 41136, 41146, 2623, 37897, 37890, 
    41126, 41107, 41129, 41425, 36522, 
    44552, 47432, 36520, 37891, 37883, 
    27368, 37884, 41132, 41118, 41120, 
    37893, 41109, 47433, 47415, 47430
  ) 
  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 0.00334

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "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": 99.9960556,
          "index_condition": "cscart_product_prices.product_id in (20838,41136,41146,2623,37897,37890,41126,41107,41129,41425,36522,44552,47432,36520,37891,37883,27368,37884,41132,41118,41120,37893,41109,47433,47415,47430)",
          "attached_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.usergroup_id in (0,1)"
        }
      }
    ]
  }
}