LOW interest rates are making it easier for homeowners to reduce their mortgage payoff times considerably. Almost a third of those who refinanced in the first quarter cut the duration of their mortgages to 15 or 20 years from 30, according to a recent refinancing report by Freddie Mac. The 31 percent who shortened their terms represented the second-highest level since 2002, when 35 percent took out shorter-term loans, the data showed. In the fourth quarter of 2011, 34 percent had reduced their mortgage terms. The all-time high occurred in 1992, with 42 percent refinancing into shorter mortgages.