Borrowers with expiring prepayment penalties are apparently taking advantage of the chance to refinance, as repayment rates have kicked upward recently. But analysts at a Wall Street investment house say that public policy might also have a hand in the speed increase. In a June 28 report, Credit Suisse First Boston reported that prepayments are speeding up on all vintages of fixed-rate subprime mortgages dating back to 1995. The most marked increase was found in