Lately some of the most popular 401(k) funds have shown signs of slowing down. After performing brilliantly in the early part of the decade, Dodge & Cox Stock has delivered uninspiring results in recent years. During the past three years, the fund has lost 10.3% annually, lagging the S&P 500 by 3 percentage points and trailing 79% of its large value peers, according to Morningstar. American Funds Growth Fund of America has lost 6.7% annually for the past three years, lagging 57% of large growth funds. While such results are troubling, proponents of the big funds argue that the situation is far from dire. None of the top 10 funds blew up during the credit crisis. That was not surprising because the top funds are run by big companies that work hard to avoid disaster with their 401(K) funds. Without exception, these popular funds are broadly diversified and take limited risks. Such cautious choices may not lead the pack, but they are unlikely to finish last either. Besides being steady, the funds that perform well tend to charge modest fees. That helps to explain why the funds outperformed during their best days. These low fees should continue providing an important edge in the future.