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 (
    125591, 125590, 125589, 125588, 125587, 
    125586, 125585, 125584, 125583, 125582, 
    125581, 125580, 125579, 125578, 125577, 
    125576, 125575, 125574, 125573, 125572
  ) 
  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 0.00070

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_prices range usergroup,product_id,lower_limit,usergroup_id usergroup 9 40 Using where

Result

product_id price
125572 1.69000000
125573 1.69000000
125574 1.69000000
125575 1.69000000
125576 1.69000000
125577 1.69000000
125578 1.69000000
125579 1.69000000
125580 1.69000000
125581 1.69000000
125582 1.69000000
125583 1.69000000
125584 1.69000000
125585 1.69000000
125586 1.69000000
125587 1.69000000
125588 1.69000000
125589 1.69000000
125590 1.69000000
125591 8.99000000