SELECT 
  q_product_prices.product_id, 
  MIN(
    IF(
      q_product_prices.percentage_discount = 0, 
      q_product_prices.price, 
      q_product_prices.price - (
        q_product_prices.price * q_product_prices.percentage_discount
      )/ 100
    )
  ) AS price 
FROM 
  q_product_prices 
WHERE 
  q_product_prices.product_id IN (
    6957, 6958, 6959, 6960, 6961, 6962, 6963, 
    6964, 6965, 6966, 6967, 6968, 6969, 
    6970, 6971, 6972, 6974, 6977, 6978, 
    6979, 4497, 4498, 4499, 4500, 4501, 
    4502, 4503, 4504, 4505, 4506, 4507, 
    4509
  ) 
  AND q_product_prices.lower_limit = 1 
  AND q_product_prices.usergroup_id IN (0, 1) 
GROUP BY 
  q_product_prices.product_id

Query time 0.00083

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "78.21"
    },
    "grouping_operation": {
      "using_filesort": false,
      "table": {
        "table_name": "q_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": 33,
        "rows_produced_per_join": 6,
        "filtered": "19.99",
        "index_condition": "(`portal`.`q_product_prices`.`product_id` in (6957,6958,6959,6960,6961,6962,6963,6964,6965,6966,6967,6968,6969,6970,6971,6972,6974,6977,6978,6979,4497,4498,4499,4500,4501,4502,4503,4504,4505,4506,4507,4509))",
        "cost_info": {
          "read_cost": "76.89",
          "eval_cost": "1.32",
          "prefix_cost": "78.21",
          "data_read_per_join": "158"
        },
        "used_columns": [
          "product_id",
          "price",
          "percentage_discount",
          "lower_limit",
          "usergroup_id"
        ],
        "attached_condition": "((`portal`.`q_product_prices`.`lower_limit` = 1) and (`portal`.`q_product_prices`.`usergroup_id` in (0,1)))"
      }
    }
  }
}