In 2001, as the current housing boom got underway, fewer than 2% of California homes were bought with interest-only loans, according to an analysis done for The Times by LoanPerformance, a San Francisco mortgage research firm. By last year, the level had risen to 48%. Nationally, LoanPerformance says, interest-only loans were used in about a third of all purchases. What's propelled the market up in California, some experts worry, could just as easily speed its descent.