When I travelled in 12779 to Delhi it was held up at the outers of ET, JHS, GWL and also NZM also it took unscheduled halt at Habibganj. From MTJ it was doing a high speed run well enough to reach NZM before time but after TKD it started to crawl and reached NZM 10 to 15 minutes late.
The problem is unscheduled halts due to whatever reasons that causes delays.