What if 3 percent is the new 8 percent?

Institutional investors such as pension funds have typically built in return assumptions of 8 percent a year - a rate some of them have not achieved for more than a decade.

Faced with slower economic growth across the world, some are cutting these long-term assumptions for the first time in a quarter of a century, with potentially far-reaching implications for pensioners, savers and asset managers.

Since the world entered what PIMCO dubbed the New Normal or slower economic growth in 2009, investors have had a relatively easy time making money thanks to cheap cash from central banks fuelling virtually all asset prices. This year alone benchmark world stocks have gained 15 percent.

But even with that, large investors, especially pension funds, have failed to achieve the 8 percent target, partly due to high fixed income holdings which returned very little.

According to the OECD, the weighted average real net investment return of pension funds which manage combined assets of over $32 trillion was 4.4 percent in 2012, and just 0.2 percent in the year before.

U.S. public pension funds, which have been increasing the share of equities in their portfolio, made an average quarterly return of 3.45 percent in the first nine months of 2013, based on data from Wilshire Associates. Wilshire's data shows the median return was 5.2 percent over the last five years.

Their score is rather disappointing, even in the investor-friendly environment of the past few years. Looking ahead to 2014, the 8 percent return target looks even less achievable, especially as the Federal Reserve starts to scale back bond buying.

"We're starting to pivot away from sugar highs from all the central bank policies to more underlying economic growth which will determine investor returns. Investors need to recalibrate as a result their asset allocation," said Alexander Friedman, global chief investment officer at UBS Wealth Management.

Friedman forecasts equities will return about 7-8 percent annually over the next three to five years.