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 (
    6303, 6304, 5801, 5802, 5800, 5799, 5803, 
    5804, 7197, 7195, 7196, 7190, 7189, 
    7194, 7191, 5032, 5033, 7674, 7675, 
    6967, 6966, 6645, 6644, 6647, 6648, 
    4352, 4354, 5391, 5392, 5388, 5390, 
    5389, 5396, 5395, 5394, 5034, 5035, 
    5030, 5031, 5168, 5169, 6982, 6132, 
    7150, 7149, 7148, 7151, 6768
  ) 
  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 0.00125

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "34.51"
    },
    "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": 50,
        "rows_produced_per_join": 9,
        "filtered": "19.12",
        "index_condition": "(`dbggbern`.`cscart_product_prices`.`product_id` in (6303,6304,5801,5802,5800,5799,5803,5804,7197,7195,7196,7190,7189,7194,7191,5032,5033,7674,7675,6967,6966,6645,6644,6647,6648,4352,4354,5391,5392,5388,5390,5389,5396,5395,5394,5034,5035,5030,5031,5168,5169,6982,6132,7150,7149,7148,7151,6768))",
        "cost_info": {
          "read_cost": "33.55",
          "eval_cost": "0.96",
          "prefix_cost": "34.51",
          "data_read_per_join": "229"
        },
        "used_columns": [
          "product_id",
          "price",
          "percentage_discount",
          "lower_limit",
          "usergroup_id"
        ],
        "attached_condition": "((`dbggbern`.`cscart_product_prices`.`lower_limit` = 1) and (`dbggbern`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
      }
    }
  }
}