Heavy scrutiny distracts NPS managers from generating higher returns on retirement money earmarked for the fastest-ageing population among advanced economies, insiders say.
Since last month, the chief investment officer of NPS has been brought before four parliamentary panels to explain and justify investment decisions - an increasingly onerous annual process that has "cut into" the work of investment managers below him, said another NPS manager.
NPS' domestic influence, and the subsequent scrutiny it faces, has grown steadily as its assets have ballooned. Set up in 1988, NPS managed 500 trillion won of assets as of end-July, compared with around 141 trillion won at the close of 2004.
"The balance between accountability of a public fund and the room to make returns is important," said an NPS spokesman.
While NPS might feel vindicated about its Southeast Asia decision last year - an MSCI index of stocks in that region has fallen about 14.6 percent in the year to Oct. 21 - it knows it needs to diversify, and it wants at least 30 percent of its assets to be held overseas by 2020.
At the end of 2014, 78 percent of NPS' holdings were invested in South Korea, even though the country accounts for less than 2 percent of global capital markets.
In contrast, the Canada Pension Plan Investment Board (CPPIB) held about 76 percent of its assets overseas as of March, while Japan's Government Pension Investment Fund had around 35 percent of its assets outside Japan as of June.
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In South Korea, NPS owns 5 percent or more of some 166 companies including Samsung Electronics and Hyundai Motor.
NPS's awkward influence came to the fore in July when it cast the deciding vote for a merger between two units of Samsung Group - a deal that many investors said short-changed minority shareholders.
In doing so, NPS bypassed an external committee, mostly of academics, charged with advising on difficult votes.
Political interference goes beyond second-guessing.
As part of a government push to decentralise from Seoul, the NPS investment office will relocate next year to the small city of Jeonju, a move that some private fund managers and insiders say makes NPS a less attractive place to work.
It is also relatively understaffed.
NPS had 261 people in its investment office as of September, managing on average about 1.9 trillion won ($1.68 billion) in assets each. By comparison, CalPERs, the largest U.S. pension fund, has about 370 people in its investment office overseeing $288 billion in assets, a CalPERS spokesman said, or $779 million each.
To address its staffing shortfall, NPS is hiring about 70 investment managers this year, and for the first time, four foreigners, for offices in London and New York, but NPS top brass still think they need more, according to an NPS spokesman.
The NPS hopes more staff means squeezing more returns out of investments.
Its 6.9 percent average annual return between 2009 and 2013 lagged the 11-13 percent rates of Norway's GPFG and the Netherlands's ABP, the world's second-and fourth-largest pension funds, according to South Korea's parliamentary budget office.
It slightly outperformed Japan's GPIF, the largest pension fund.
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NPS said it is a long-term investor and its performance should be judged accordingly.
But the clock is ticking - one in five South Koreans will be 65 or over by 2026.
In 2043, the value of NPS assets is expected by the government to peak at 2,561 trillion won before declining, as payouts overtake fund inflows, at current distribution rates.
Last year, NPS reported a modest return rate of 5.25 percent, partly because of a negative 5.4 percent return on the Korean stocks held by NPS.
"There is no clear public consensus on how to manage the national pension scheme," said Shim Ji-heon, a researcher at the National Assembly Budget Office.
"We need to start a public discussion on whether to leave the fund's inflow and outflow levels as they are, or to rethink the level of risk the pension is willing to take on."