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 (
    15087, 15056, 15050, 15049, 15048, 15047, 
    11134, 2816, 2815, 13609, 13514, 2214, 
    15387, 2451, 2453, 2162, 12741, 11785, 
    15298, 2806, 2805, 2710, 2709, 2712, 
    2711, 4211, 4353, 11891, 11835, 2253
  ) 
  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 0.01279

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": 7.156754494,
          "attached_condition": "cscart_product_prices.lower_limit <=> 1 and cscart_product_prices.product_id in (15087,15056,15050,15049,15048,15047,11134,2816,2815,13609,13514,2214,15387,2451,2453,2162,12741,11785,15298,2806,2805,2710,2709,2712,2711,4211,4353,11891,11835,2253) and cscart_product_prices.usergroup_id in (0,1)"
        }
      }
    }
  }
}