SELECT 
  cscart_product_prices.product_id, 
  COALESCE(
    cscart_master_products_storefront_min_price.price, 
    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 
  LEFT JOIN cscart_master_products_storefront_min_price ON cscart_master_products_storefront_min_price.product_id = cscart_product_prices.product_id 
  AND cscart_master_products_storefront_min_price.storefront_id = 1 
WHERE 
  cscart_product_prices.product_id IN (
    20589, 20590, 20669, 20597, 20595, 20593, 
    20594, 20693, 20596, 20672, 20670, 
    20671, 20614, 20615, 20616, 20673, 
    20617, 20618, 20600, 20715, 20602, 
    20716, 20694, 20717
  ) 
  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 0.00122

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_master_products_storefront_min_price",
      "access_type": "system",
      "possible_keys": ["PRIMARY"],
      "rows": 0,
      "filtered": 0,
      "const_row_not_found": true
    },
    "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": 24,
      "filtered": 75,
      "index_condition": "cscart_product_prices.product_id in (20589,20590,20669,20597,20595,20593,20594,20693,20596,20672,20670,20671,20614,20615,20616,20673,20617,20618,20600,20715,20602,20716,20694,20717)",
      "attached_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.usergroup_id in (0,1)"
    }
  }
}

Result

product_id price
20589 1990.00000000
20590 1990.00000000
20593 590.00000000
20594 690.00000000
20595 590.00000000
20596 590.00000000
20597 690.00000000
20600 690.00000000
20602 590.00000000
20614 690.00000000
20615 790.00000000
20616 690.00000000
20617 690.00000000
20618 690.00000000
20669 1990.00000000
20670 590.00000000
20671 590.00000000
20672 590.00000000
20673 590.00000000
20693 690.00000000
20694 690.00000000
20715 690.00000000
20716 690.00000000
20717 690.00000000