Three years after the start of the credit crisis sparked by mortgage-backed securities, a top-performing investment has been … mortgage-backed securities. But only those backed by the government or packaged by Fannie Mae or Freddie Mac. The average mutual fund that invests in mortgage-backed securities issued by Ginnie Mae has gained 24.8% the past three years, including reinvested interest, vs. 20.2% for funds that invest in Treasury securities, according to Lipper. The Standard & Poor’s 500-stock index has fallen 20.8% since Aug. 31, 2007.