Some leading retail Web sites struggled to keep up with the rush of cybershoppers on Black Friday and Cyber Monday, two of the busiest online shopping days, according to Keynote Systems, which monitors Web site performance.

According to Keynote, some of the sites that experienced technical difficulties were those of retailers Buy.com, Eddie Bauer, J.Crew and Toys R Us Keynote said it measured the technical performance of 30 leading online retailers.

"We're seeing two overarching trends so far this holiday season," Shawn White, director of external operations for Keynote, said in a statement issued by Keynote. "First, is that there appear to be significant slowdowns on a number of leading retail Web sites, with traditional brick-and-mortar retailers faring the worst. Second, is the fact that the slowdowns are impacting at the crucial product search and checkout portions of their shopping process."

According to Keynote, it took some consumers up to two minutes to process their transactions -- even if they had high-speed Internet connections. It usually only takes one to two seconds to process transactions, Keynote said.

The worst-performing sites on Black Friday were sometimes four times slower than normal, Keynote said, adding that shoppers experiencing such slowdowns would likely give up and buy the product on another Web site.

Keynote said most of the sites having problems were those of retailers that have only recently upgraded their online presence, like Costco Wholesale and Lowe's Companies.

At the other extreme, the sites of Best Buy Co., Barnes & Noble, Dell and Overstock.com performed well despite the heavy traffic, Keynote said.

The Toys R Us Web site was three times slower than usual, Keynote said. From 1 p.m. to 5 p.m. Eastern time on Monday, it took an average of 30 to 60 seconds to download, according to Keynote. The slowdowns affected the home page as well as a shopper's ability to search for a product or add that product to a shopping cart, Keynote said.

On Cyber Monday, Costco's Web site was five times slower than normal, causing shoppers to have problems trying to add items to their shopping carts and complete their purchases, Keynote said. It sometimes took 115 seconds -- compared to 20 seconds on a normal day -- for Web pages to download.

Buy.com had significant problems with its Checkout button on Cyber Monday, Keynote said.