Black Diamonds To Speed Up Adventure? Or Slow It Down?

I have noticed that there's a new feature on the Adventure where you can use black diamonds to speed up what is normally a 3 day journey on average. HOWEVER, I've also noticed that since this was implemented, the journey's are taking much longer. In fact, I have not finished a journey on time since this started, and I never missed one until this started.

What's up with that?

~Dragon Queen