Mortgage-backed securities prepayment speeds slowed down in August, the result of rising mortgage rates in the early- to mid-summer, and Wall Street analysts expect further deceleration in coming months. The prepayment rate on Fannie Mae’s 30-year, 5.5 percent coupon bonds dropped in August to 33.5 percent from a record of 45.1 percent in July. Freddie Mac’s 5.5 percent coupon bonds prepaid at 37.3 percent last