Web traffic increases on Black Friday and Cyber Monday should be no surprise, but one-third of major online retailers were still unprepared, according to a study that illuminates ongoing problems within the e-commerce industry.

“Every year I am really surprised that certain sites have outages or performance issues because online shopping is not a new thing,” says Shawn White, director of external operations for Keynote, a vendor that continuously tracks Web site performance of the top 30 retail companies. One-third of them suffered significant slowdowns on Black Friday and Cyber Monday.

The worst-performing retailers were traditional “bricks-and-mortar” stores that only recently invested heavily in the Web, according to Keynote. The Toys “R” Us page was three times slower than usual for several hours on Cyber Monday, with page download times sometimes exceeding one minute, Keynote said.

Searching for products and adding items to the customer’s shopping cart were the two hardest tasks. J. Crew, Lowe’s and Costco experienced similar slowdowns, particularly Costco. Page download times that typically take 20 seconds on the Costco site were averaging as high as 115 seconds on Cyber Monday, Keynote says.

“A lot of these traditional brick-and-mortar companies, online retailing is not their core competency,” White says. “There’s a lot to learn from your traditional online retailers, such as Amazon and Barnes & Noble, and what they’ve been doing with online shopping for years.”

One exception was Buy.com, which performed poorly despite being online-only.

Preparation for big online shopping days should begin months beforehand, White says. Cyber Monday can be challenging in any year, and traffic in the morning hours was up 18% over last year’s Monday after Thanksgiving, according to Akamai,  which tracks global retail usage.

Keynote, which performs load testing to examine whether customers’ Web sites can handle big increases in traffic, begins working with customers as many as eight months before the holiday season “onslaught,” White says.

Retailers that perform poorly in testing often need more bandwidth capacity, better load balancing or a distributed server infrastructure, he says.

Most of this year’s problems occurred on Black Friday from 10 a.m. EST until 3 p.m., and on Cyber Monday from 11 a.m. until 3 p.m., White says.