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 (
    11059, 6813, 6919, 6920, 6921, 6923, 
    6922, 11211, 10640, 8865, 21995, 22344, 
    21996, 20369, 20370, 20371, 8000, 6704, 
    6740, 6751, 6769, 6743, 6777, 6781
  ) 
  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 0.01138

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": "ref",
              "possible_keys": [
                "usergroup",
                "product_id",
                "lower_limit",
                "usergroup_id"
              ],
              "key": "lower_limit",
              "key_length": "3",
              "used_key_parts": ["lower_limit"],
              "ref": ["const"],
              "rows": 11706,
              "filtered": 10.04527187,
              "attached_condition": "cscart_product_prices.lower_limit <=> 1 and cscart_product_prices.product_id in (11059,6813,6919,6920,6921,6923,6922,11211,10640,8865,21995,22344,21996,20369,20370,20371,8000,6704,6740,6751,6769,6743,6777,6781) and cscart_product_prices.usergroup_id in (0,1)"
            }
          }
        ]
      }
    }
  }
}