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 (
    26977, 6554, 15218, 41399, 25730, 47599, 
    2015, 2255, 6098, 48826, 40526, 6555, 
    25180, 286, 4071, 26854, 405, 5226, 
    26604, 294, 33718, 2576, 26608, 26864, 
    2414, 21188, 2797, 2492, 26846, 2302, 
    6523, 38560
  ) 
  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 0.06155

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": 25327,
              "filtered": 8.333330154,
              "attached_condition": "cscart_product_prices.lower_limit <=> 1 and cscart_product_prices.product_id in (26977,6554,15218,41399,25730,47599,2015,2255,6098,48826,40526,6555,25180,286,4071,26854,405,5226,26604,294,33718,2576,26608,26864,2414,21188,2797,2492,26846,2302,6523,38560) and cscart_product_prices.usergroup_id in (0,1)"
            }
          }
        ]
      }
    }
  }
}