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 (
    6521, 6508, 6419, 6519, 6420, 6517, 6458, 
    6597, 6565, 6566, 6570, 6599, 6568, 
    6596, 6598, 6595, 6273, 6265, 6329, 
    6357, 6350, 6348, 6322, 6285, 6327, 
    6283, 6284, 6326, 6363, 6351, 6324, 
    6409, 6863, 6325, 6328, 6349, 6353, 
    6410, 6418, 6352
  ) 
  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 0.00165

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "28.01"
    },
    "grouping_operation": {
      "using_filesort": false,
      "table": {
        "table_name": "cscart_product_prices",
        "access_type": "range",
        "possible_keys": [
          "usergroup",
          "product_id",
          "lower_limit",
          "usergroup_id"
        ],
        "key": "product_id",
        "used_key_parts": [
          "product_id"
        ],
        "key_length": "3",
        "rows_examined_per_scan": 40,
        "rows_produced_per_join": 7,
        "filtered": "19.99",
        "index_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`cscart_product_prices`.`product_id` in (6521,6508,6419,6519,6420,6517,6458,6597,6565,6566,6570,6599,6568,6596,6598,6595,6273,6265,6329,6357,6350,6348,6322,6285,6327,6283,6284,6326,6363,6351,6324,6409,6863,6325,6328,6349,6353,6410,6418,6352))",
        "cost_info": {
          "read_cost": "27.21",
          "eval_cost": "0.80",
          "prefix_cost": "28.01",
          "data_read_per_join": "191"
        },
        "used_columns": [
          "product_id",
          "price",
          "percentage_discount",
          "lower_limit",
          "usergroup_id"
        ],
        "attached_condition": "((`softwarepirmam_hewadelivard_cscart_4`.`cscart_product_prices`.`lower_limit` = 1) and (`softwarepirmam_hewadelivard_cscart_4`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
      }
    }
  }
}