A looming economic slowdown could spell bad news for the performance of some home-equity securitizations, a new report from a Wall Street investment bank suggests. In fact, according to the Credit Suisse First Boston analysis, signs of trouble may already be surfacing. The recent CSFB report noted an upswing in serious delinquencies for nearly all vintages of loans backing subprime securitizations issued since 1995. From an average of 11.84 percent at the end of September,