For 25 large online retailers, Gomez measures their sites' ability to complete transactions, from picking a product to paying for it, and found that Victoria's Secret fared particularly badly on Monday. That day, the so-called Cyber Monday, has been seen as a big day for e-commerce following big Thanksgiving weekend shopping sprees at brick-and-mortar stores.

Between 6 a.m. and midnight U.S. Eastern time Monday, the Victoria's Secret Web store had availability of 74.30 percent, meaning that out of every 100 visitors, almost 26 were unable to complete a transaction due to Web site problems, Mouline said. Most issues were centered on the site's shopping cart, which for several hours malfunctioned, not allowing shoppers to place items in it, he said.

Others that had availability problems on Monday included Staples (88.32 percent), Costco (92.45 percent), Dell (95.26 percent) and OfficeMax (96.26 percent).

Any performance degradation is costly to online retailers because every day, shoppers are savvier and have higher expectations for convenience, especially when search engines and comparison shopping sites make it increasingly easy to find the same product elsewhere on the Web, he said.

Limited Brands, the parent company of Victoria's Secret, didn't respond to requests for comment on Monday and Tuesday. A Dell spokesman said Monday that the company hadn't detected any problems with its Web site. Staples, Costco and OfficeMax didn't immediately respond to requests for comment.