You are right trains can be speeded up by reducing slack time but at the same time IR needs to take care of freights which are cross subsiding passenger fares and also the signalling systems needs to updated at every junction. In am from Visakhapatnam and when Janmabhoomi exp leaves @615 there ate freights waiting at outer to join mainline and after passing 20-35 km it has to wait for clearance of ongoing freight to get to loop.