The reason for this is that the monthly repayment of equal principal and interest is the same, while the monthly repayment of average capital is more first and then less. The monthly repayment amount is directly proportional to the declared monthly income (generally not more than half of the monthly income).
Whether it is cost-effective or not is the same. Monthly repayment consists of monthly interest and part of principal. Pay interest first, then pay the principal.
Monthly interest rate = monthly interest rate * loan principal balance; Repay the principal in part according to the repayment method.
It doesn't make much difference to pay off in six years.