If stocks return an average of 10% per year over the next 25 years, a $10,000 investment would turn into $108,347. But if average annual returns fell to 8%, that $10,000 would be worth $68,485. Over a quarter century, a 2-percentage-point difference in annual returns leaves you with portfolio nearly 40% smaller than it would have been otherwise. If returns average just 5% per year, the scenario becomes even bleaker: $10,000 becomes just $33,864.