Interestingly... too fast can be a problem too. A prime example is this site, Hacker News, on a really fast browser. I'll often sit there waiting for a navigation to load, until I realize it had already loaded (to a similar looking page), just so fast I didn't notice the transition.

That’s why animations are important contrary to what many say here in the comments (and I agree that putting them here and there just for the sake of it is bad). They give “life” to a virtual object. For example I really like tiling window managers, but they could use a really fast animation for window switches because the changing number at the top of the screen doesn’t say as much as movement to my primate brain.