I just pulled up the 10 year annual rate of return (compounded) on the TSP. The C fund, which tracks the S&P 500, has an annual return of 2.94% over that period. That compares to the G fund's return of 3.96%, and the S fund's return of 6.76%. Essentially, you would have been better off in government securities than in the S&P 500 over the past 10 years.
The private sector bond basket (the F fund) has beat the C fund 3 of the past 5 years. The only two years the C Fund beat it were in 2009 and 2010, which were the years after the S&P 500 lost > 1/3 its value (2008), so that return has to be taken with some salt.
Needless to say, I need to stare very hard to find why the heck I bother trying to invest my retirement savings in anything other than drug shipments and lottery tickets.