The TSP does a great job of educating users. However, it never publishes 15-year returns for the funds. 

That's too bad. The ten-year returns don't include the effects of the 2007-2009 market crash, which caused the C Fund to decline 34%. The 15-year returns do include that, so they offer a better picture of long-term returns.

The 15-year returns are the last line of this table:

Over the last 15 years, the US stock funds outperformed the bond funds by a significant amount, despite large declines (bear markets) in 2007-2009 and 2020. The F Fund outperformed G by 84%. I Fund performance was poor.

Notes:

This is for educational purposes only. To learn more about the topics mentioned and if they are suitable for you, consult an appropriate professional. Tax laws can change at any time. Any information provided in this presentation has been prepared from sources believed to be reliable, but is not guaranteed and is not a complete summary or statement of all available data necessary for making an investment decision. Any information provided is for information purposes only and does not constitute a recommendation. Past performance is no guarantee of future performance. Investments involve the risk of loss of principal and earnings. “Average annual return” evens out variations in the actual year-to-year returns. Circumstances differ among individuals and they should not assume that these generalizations or information apply to them. Investments mentioned may not be suitable for all investors