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 (
    2556, 2822, 2244, 2245, 2808, 2351, 2352, 
    2342, 2270, 2843, 2225, 2323, 2266, 
    13822, 2529, 2503, 2813, 2814, 13821, 
    4378, 2550, 2551, 13142, 15143
  ) 
  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 0.00763

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "cscart_product_prices.product_id",
      "temporary_table": {
        "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": 6449,
          "filtered": 4.219423294,
          "attached_condition": "cscart_product_prices.lower_limit <=> 1 and cscart_product_prices.product_id in (2556,2822,2244,2245,2808,2351,2352,2342,2270,2843,2225,2323,2266,13822,2529,2503,2813,2814,13821,4378,2550,2551,13142,15143) and cscart_product_prices.usergroup_id in (0,1)"
        }
      }
    }
  }
}