One of the main reasons is too much traffic at some particular time of the day. e.g. Congestion at CNB towards midnight. Such congestion cannot be dealt with the freight corridor unless trains timetable is refreshed and designed in a way to avoid that congestion keeping factors like delay of trains etc in mind.
There have been times in this month when NCR performed unexpectedly very good with only 1-2 Delhi reaching trains getting delayed. Almost all 130,110 kmph trains were on time. This means that they are really trying to improve things. But yes, sometimes stopping trains at outer despite platform availability is awkward. Also, unnecessary extra priority to some trains hard to understand. e.g. 12260 SDAH Duronto is kept behind...
more... Shivganga till ALD these days. But considering overall efforts and the fact that after joining of Mr S.K. Jain, the amount of train delay has reduced, i would give you a NO for the answer although at times, while tracing trains on NTES i get extremely annoyed over NCR,