Maybe I'm a minority, but I can't really trust anything from a company with a login screen that says "logging you in securely" for 5000ms before loading me (sometimes) to my dashboard.
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.