I guess IR has kept speeds on NDLS-CNB route lesser due to the immense over utilization of these tracks. The same reason why so many Rajdhanis and other trains crowd these tracks makes them slower in the long run.Also, GZB-NDLS is a slower section comparatively.
I can understand your sentiment regarding slowing down of trains. The Lucknow Shatabdi once used to do 6 hrs 15 mins, I think back when it had a non stop run except at CNB. Now it takes 6hrs 55 mins stopping even at small stations like Phaphund.