In all preceding Mario Kart games, using a Star to become invincible grants an increase in top speed for the duration of the effect, which is arguably more useful than the invincibility part.
In Mario Kart 8, it really doesn't feel like this is the case. It seems that invincible players move at the same speed as everyone else. This makes the item pretty worthless when it comes to attempting to pass people, since not only can you not just pass them, but you can't catch up to them to ram them.
I'd like to know whether this is the effect of coins on top speed being stronger than expected (as people who get Stars tend to have not many coins), or whether the speed boost has actually been removed (as yet another baffling design choice).