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 (5, 6, 14) 
  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 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "vi_product_prices",
          "access_type": "range",
          "possible_keys": [
            "usergroup",
            "product_id",
            "lower_limit",
            "usergroup_id"
          ],
          "key": "product_id",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "rows": 3,
          "filtered": 100,
          "index_condition": "vi_product_prices.product_id in (5,6,14)",
          "attached_condition": "vi_product_prices.lower_limit = 1 and vi_product_prices.usergroup_id in (0,1)"
        }
      }
    ]
  }
}