SEATTLE — Sears.com was inaccessible to U.S. shoppers for two hours on Friday in what was the most notable Web hiccup of the holiday gift-buying season’s official start.

Other sites, including Amazon.com Inc., experienced minor slowdowns, according to Shawn White, director of external operations at Keynote Systems Inc., a San Mateo, Calif.-based research group.

Starting a week and a half ago, Keynote began tracking the performance of about 30 big online retailers, logging the time it took to find a product and start checking out.

Keynote’s list includes Wal-Mart Stores Inc., Macy’s Inc., Circuit City and others; the system takes measurements every 15 minutes from computers in 10 major U.S. cities.

Sears Holdings Corp.’s site started to crawl at around 9:30 a.m. Eastern time on Friday, when loading a page on the site topped one minute. From about 10:30 to 12:30, Sears posted a message asking shoppers to try again in a few minutes.

White said Sears was among the retailers that stumbled last year on Black Friday.

But while Sears’ problems returned this year, others, including Neiman Marcus and Buy.com Inc., seem to have resolved past issues.

Amazon and Target Inc., which uses Amazon’s e-commerce technology, were slower Friday than in recent days, but not unbearably so, White said. At the slowest point, a transaction that took 25 seconds last week required about 40 seconds Friday morning.

Kohl’s Corp. and Saks Inc. also had performance problems, according to Keynote data.

White said he expects some sites will slow down or shut down on Monday, too, as workers, back in the office after the holiday weekend, start clicking.