This might provide you some insight: http://thedailywtf.com/Articles/The-Slow-Down-Loop.aspx

In all seriousness, adding interstitials can provide value. If the login performance has high variance, it's better to set an expectation that it will take 5 seconds than leave the user wondering if the click actually worked. Similarly, Apple has done an amazing job with UI transitions in iOS to mask the actual load time.