Microsoft declined to comment on Pfeiffer's Vista user interface research.

In the common desktop task benchmark, which gauged how long it took users to open a folder, delete files, and so on, Vista running Aero was 14 percent slower than XP. The final benchmark of mouse precision, a test crucial to design professionals and photographers, but also of interest to general users who get frustrated trying to nail submenu commands at the first click, also put Vista on the bottom. Pfeiffer's Vista "mouse precision coefficient" was 30 percent higher than XP's. A higher coefficient means users found it harder to precisely place the mouse.

"These things are very measureable," Pfeiffer said. "In Vista, a folder fades in, as if it appears out of nothing. It looks great, but after 10 times you realize you're losing time waiting for that."

Switching to the Basic or Classic interfaces, used primarily by low-powered systems and corporations that want interface consistency, respectively, improves Vista scores, but at a sacrifice of one of the OS's most compelling features. Dropping down to Classic gives Vista a menu latency score similar to XP's, but on desktop operations, it still lags behind the older operating system.

In every benchmark, Windows scored significantly poorer than Mac OS X, which is far more "fluid" than Microsoft's OSes, according to Pfeiffer.