In addition to availability, Gomez measures response time, which is the time it takes a Web site to complete all the steps in a purchase transaction. By this measure, the site that fared the worst was that of Williams-Sonoma, with an average response time of 27.43 seconds. It was followed by Target, with 21.16 seconds, and Costco, with 21 seconds.

A Williams-Sonoma spokeswoman said via e-mail on Monday: "We are happy to report that our customers have had no trouble shopping our e-commerce site. Traffic is strong and we have not experienced any delays."

Target and Costco didn't immediately respond to requests for comment.

As a point of comparison, the 25 retail Web sites for which Gomez measures transaction performance had an average response time of 17 seconds and availability of 98.13 percent in October.