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 (
    26863, 43796, 4237, 43798, 5270, 5257, 
    6070, 37849, 335, 5726, 5288, 40809, 
    5228, 18194, 5265, 40754, 4279, 40807, 
    926, 1266, 26851, 32360, 1542, 6103, 
    26857, 37942, 2449, 31904, 31618, 6102, 
    31646, 35735
  ) 
  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 0.00186

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_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": 32,
          "filtered": 99.9960556,
          "index_condition": "cscart_product_prices.product_id in (26863,43796,4237,43798,5270,5257,6070,37849,335,5726,5288,40809,5228,18194,5265,40754,4279,40807,926,1266,26851,32360,1542,6103,26857,37942,2449,31904,31618,6102,31646,35735)",
          "attached_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.usergroup_id in (0,1)"
        }
      }
    ]
  }
}