Tim Murray, director of Parallel, a UK-based network management firm, said his company's Nexus Watch service found the current mean availability of FTSE 100 companies' Web sites was 99.53% a year. This represents 41 hours of downtime.
The worst sites, he said, were in the logistics support services sector, where site availability was only 98.39%.
The best performer, in terms of availability was in the insurance sector, with an uptime of 99.97%.
"This is quite respectable," said Murray, "given that five nines availability [99.999%] isn't yet possible on the Internet."
One of the easiest ways to improve performance and reliability is to simplify site design, Murray advised. "Web designers recommend that Web site home pages should be less than 40Kbytes in size. But the average among FTSE 100 Web sites was 60Kbytes." With a 30Kbytes homepage, Murray said, the average download time would be in the region of eight seconds.