Copying files is an interesting case study. Vista was actually faster but perceived to be slower. XP wouldn't actually tell you when copying was 100% complete. If you turned off your power supply the second XP's copy dialog went away you'd lose data.

Vista sped up file copying operations but fixed that bug, leading to a faulty perception of slowness. Worse, the progress bar behavior encouraged that perception of slowness. A progress bar that speeds up at the end will be perceived to be faster than one that is perfectly even which in turn is perceived faster than one that slows down at the end. And vista's progress bar usually slowed down at the end because it didn't properly account for those disk sync et al operations ahead of time. The result was a worse feeling experience despite what was happening under the hood.