Keynote Systems is a company which tracks the performance of Web sites. Unexpected high demand for online merchants has led to slow performance and more frequent site failures at the beginning of December.

The 12 Web stores that Keynote recently indexed were JC Penney, OfficeMax, Sear, Nordstrom,, Costco, Best Buy, LL Bean, Eddie Bauer, Office Depot, Wal-Mart, and Target.

Keynote says that the companies are losing between 1 and 10% of business as shoppers abandon their shopping carts, or as the purchase fails.