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 (
    291, 292, 293, 294, 295, 296, 297, 298, 
    300, 302, 303, 304, 305, 306, 307, 308, 
    309, 310, 311, 312, 313, 314, 315, 316, 
    317, 318, 319, 320, 321, 330, 331, 332, 
    333, 334, 335, 338, 339, 340, 341, 342, 
    344, 345, 346, 347, 348, 349, 350, 351, 
    352, 353, 354, 355, 356, 357, 358, 359, 
    360, 361, 362, 363, 364, 365, 366, 367, 
    368, 369, 370, 371
  ) 
  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 0.00123

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "cscart_product_prices.product_id",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "cscart_product_prices",
              "access_type": "ALL",
              "possible_keys": [
                "usergroup",
                "product_id",
                "lower_limit",
                "usergroup_id"
              ],
              "rows": 78,
              "filtered": 87.17948914,
              "attached_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.product_id in (291,292,293,294,295,296,297,298,300,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,330,331,332,333,334,335,338,339,340,341,342,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,370,371) and cscart_product_prices.usergroup_id in (0,1)"
            }
          }
        ]
      }
    }
  }
}