SELECT 
  prices.product_id, 
  prices.lower_limit, 
  usergroup_id, 
  prices.percentage_discount, 
  IF(
    prices.percentage_discount = 0, 
    prices.price, 
    prices.price - (
      prices.price * prices.percentage_discount
    )/ 100
  ) as price 
FROM 
  cscart_product_prices prices 
WHERE 
  prices.product_id = 7 
  AND lower_limit > 1 
  AND prices.usergroup_id IN (0, 0, 1) 
ORDER BY 
  lower_limit

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "ordering_operation": {
      "using_filesort": true,
      "table": {
        "table_name": "prices",
        "access_type": "ref",
        "possible_keys": [
          "usergroup",
          "product_id",
          "lower_limit",
          "usergroup_id"
        ],
        "key": "usergroup",
        "used_key_parts": [
          "product_id"
        ],
        "key_length": "3",
        "ref": [
          "const"
        ],
        "rows": 1,
        "filtered": 100,
        "index_condition": "((`_testcscart`.`prices`.`lower_limit` > 1) and (`_testcscart`.`prices`.`usergroup_id` in (0,0,1)))",
        "attached_condition": "((`_testcscart`.`prices`.`product_id` <=> 7))"
      }
    }
  }
}