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 (
    1515, 1514, 1512, 2708, 4555, 421, 1967, 
    423, 422, 4558, 2723, 7735, 7734, 6193, 
    7737, 6195, 6194, 7736, 7750, 6196, 
    4415, 4417, 4414, 4412, 4416, 7588, 
    7589, 7590, 7591, 7592, 7240, 7237, 
    7236, 7238, 7239, 5875, 5877, 5874, 
    5879, 5880, 5873, 5876, 5878, 5881, 
    5872, 3999, 4004, 4005
  ) 
  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 0.00060

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "35.41"
    },
    "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": 52,
        "rows_produced_per_join": 9,
        "filtered": "19.12",
        "index_condition": "(`dbggbern`.`cscart_product_prices`.`product_id` in (1515,1514,1512,2708,4555,421,1967,423,422,4558,2723,7735,7734,6193,7737,6195,6194,7736,7750,6196,4415,4417,4414,4412,4416,7588,7589,7590,7591,7592,7240,7237,7236,7238,7239,5875,5877,5874,5879,5880,5873,5876,5878,5881,5872,3999,4004,4005))",
        "cost_info": {
          "read_cost": "34.42",
          "eval_cost": "0.99",
          "prefix_cost": "35.41",
          "data_read_per_join": "238"
        },
        "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)))"
      }
    }
  }
}