Chris Harrison et al. has a paper showing that people perceive time as running faster if they are looking at an appropriately accelerating progress bar. It seems that a progress bar that slows in the beginning and accelerates near the end (called "Fast Power" in the paper) makes people happiest. The study focused on the duration of 5.5 seconds and suggested that other longer durations should be further investigated.
I never asked myself how a progress bar should move to please people. I'm glad there are people like Chris who thinks of these things. He also has a lot of interesting projects here.
On a slightly related news, here's an article about how to slow your perception of time.