Energy of electrons in npn transistor
The electric field between the CB junction and the Collector contact pulls the electrons to the Collector contact because it has a positive voltage compared to the Base and CB junction. The electrons loose potential energy as they travel from CB junction to the Collector contact.

Perhaps you are thinking about the speed of the electrons but that is irrelevant. You should think about the potential energy instead. Pulling an electron back to the CB junction would work against that electric field so would cost energy. So an electron traveliing to the collector contact looses potential energy.