Between 6 p.m. and 8 p.m. U.S. Eastern Time, the availability for the news sites from ABC, CBS and the LA Times dropped to almost 10 percent, meaning that about nine out of 10 visitors couldn't get the sites to load.

Starting at 5:30 p.m., the average download speed for news sites tracked by Keynote went from less than four seconds to almost 9 seconds, and their average availability dropped from almost 100 percent to 86 percent, the company said. News sites monitored by Keynote returned to normal performance and availability levels by 9:15 p.m.

Other news sites that experienced problems included AOL, MSNBC, NBC, the San Francisco Chronicle and Yahoo News, according to Keynote.