SELECT 
  vi_product_prices.product_id, 
  MIN(
    IF(
      vi_product_prices.percentage_discount = 0, 
      vi_product_prices.price, 
      vi_product_prices.price - (
        vi_product_prices.price * vi_product_prices.percentage_discount
      )/ 100
    )
  ) AS price 
FROM 
  vi_product_prices 
WHERE 
  vi_product_prices.product_id IN (
    12, 78, 79, 11, 41, 81, 107, 109, 91, 92, 
    34, 15, 31, 38, 80, 102, 28, 280, 282, 
    278
  ) 
  AND vi_product_prices.lower_limit = 1 
  AND vi_product_prices.usergroup_id IN (0, 1) 
GROUP BY 
  vi_product_prices.product_id

Query time 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "vi_product_prices.product_id",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "vi_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": 255,
              "filtered": 100,
              "attached_condition": "vi_product_prices.lower_limit <=> 1 and vi_product_prices.product_id in (12,78,79,11,41,81,107,109,91,92,34,15,31,38,80,102,28,280,282,278) and vi_product_prices.usergroup_id in (0,1)"
            }
          }
        ]
      }
    }
  }
}