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 (
    4970, 4963, 4974, 4978, 4956, 4962, 4959, 
    12457, 11777, 12268, 12270, 12271, 
    12272, 12275, 12276, 12255, 12257, 
    12258, 12259, 12260, 12261, 12262, 
    12269, 12263, 12264, 12265, 12266, 
    12267, 12273, 12274
  ) 
  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 0.00774

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": 4.305534363,
          "attached_condition": "cscart_product_prices.lower_limit <=> 1 and cscart_product_prices.product_id in (4970,4963,4974,4978,4956,4962,4959,12457,11777,12268,12270,12271,12272,12275,12276,12255,12257,12258,12259,12260,12261,12262,12269,12263,12264,12265,12266,12267,12273,12274) and cscart_product_prices.usergroup_id in (0,1)"
        }
      }
    }
  }
}