The big test of retailers' Web sites occurred in the days following Thanksgiving, and a number of top retailers at times couldn't keep up with the heavy loads, according to Web monitoring firms.

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In particular, a number of retailers struggled with availability issues on Black Friday, the day after Thanksgiving.

Costco averaged roughly 64% availability between 3 p.m. and 8 p.m. (ET) on Friday before significantly improving performance on Saturday, when it delivered more than 91% availability, reports Gomez, which tracks the Top 25 online retailers’ site performance. Sears similarly suffered availability problems, averaging just over 61% availability for Friday. Sears also turned things around on Saturday, when it delivered 100% performance, Gomez says.

HP Shopping and Overstock.com suffered from sluggish response times on Friday -- nearly 45 seconds on average for HP Shopping and 17 seconds for Overstock. But both returned to normal levels (13 seconds for HP Shopping and 5 to 6 seconds for Overstock) on Saturday.

On Dec. 1, “Cyber Monday,” the Web sites of Dell, Victoria’s Secret and Williams-Sonoma experienced issues during the morning hours, according to Gomez.

Victoria’s Secret shoppers trying to place items in a shopping cart received a site unavailable page and were unable to proceed during the morning of Dec. 1, Gomez reports. Meanwhile, at Williams-Sonoma’s shopping cart/view payment pages, a typical transaction slowed from 26 seconds at 6 a.m. to almost 50 seconds between 7 a.m. and 9 a.m. At Dell’s site, returning shoppers were briefly (between 9:40 a.m. and 10:15 a.m.) unable to retrieve pre-populated account information, meaning they had to start-over filling out forms when checking out, Gomez reports.

Despite the site hiccups, online retailers managed strong sales in the days following Thanksgiving.

Black Friday saw $534 million in online spending, up 1% over last year, according to Web measurement firm comScore. For the Saturday and Sunday following Thanksgiving, online sales came in at $769, a strong 19% gain relative to last year. For Cyber Monday, comScore says online sales hit $846 million, a gain of 15%. Overall, the four-day period from Black Friday through Cyber Monday saw e-commerce spending jump 13%.

Ann Bednarz is associate news editor at Network World.