If your portfolio has an annual growth rate of 8% per year, and for 30 years you put away 5% of your income into your retirement savings, you may retire with almost half the income that you were making before retirement. If you save 10% per year of your income per year, you will be very comfortable in retirement. But if the portfolio’s inflation adjusted annual return is only 2%–3% per year, you would need to put away almost 40% of your current annual income, just to have half the income you are currently earning in retirement.